In battle, Susie's UltimatHeal move is tied to her magic stat like most other spells, and consequently recovers more HP should the player boost her stats through hacking. However, in the cutscene where she first demonstrates it to Kris, it is hard-coded to only heal 2 HP regardless of circumstance.
Contributed by game4brains
On Twitter, Toby Fox revealed that he didn't know how people would receive the character of Spamton, and he even thought that he would be negatively received. His fears were exacerbated after the character got a disdainful and disgusted reaction from test players. However Spamton, nonetheless, turned out to be popular with fans and even garnered his own cult following with the massive sales and popularity of his merchandise not being able to meet demand at times.
Contributed by PirateGoofy
The phone in Spamton's shop is hard-coded to ring in time with the shop's background music, "Dialtone". Hacking the game to turn off the song results in the phone remaining motionless indefinitely.
Contributed by game4brains
In the livestreamed playthrough of Chapter 1 conducted for Undertale's sixth anniversary, Toby Fox stated that he originally envisioned the game having a greater visual contrast between the Light World and Dark World, with the former looking "drab" and simplistic and the latter having more vibrant, detailed graphics in the vein of Chrono Trigger or Super Mario RPG: Legend of the Seven Stars.

Similarly, the Light World was at one point planned to be populated entirely by humans, and the Dark World entirely by monsters, with humans turning into monsters when they enter the latter. According to Fox, the idea was removed for "a lot of reasons," most prominently a desire to better distinguish the project from Undertale.
Contributed by game4brains
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Within the data for Chapter 2 are unused graphics for an alternate, more detailed version of the scene where Ralsei heals Susie after the first Berdly fight. Among other things, these graphics depict Susie's hand bleeding, deviating from the established portrayal of monsters being made entirely from magic (and consequently lacking blood); it's likely that this contradiction is the reason why the graphics went unused.
Contributed by game4brains
It's possible to close the pop-up windows that appear during a Poppup's attacks using the mouse, functioning identically to closing an actual computer window.
Contributed by game4brains
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In Chapter 2, Kris' eye can briefly be seen during the cutscene that plays whenever they and Susie enter a Dark World, appearing just before the pair plunge down at the end of the transition. This appears to nod to a graphical quirk present since Chapter 1, where the pixels in Kris' sprite will occasionally blend together while walking around, resulting in a red dot sometimes appearing in roughly the same area of their face by mistake.
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Within the data for Chapter 2 is an unused alternate version of Spoiler:Spamton NEO's wasted turn, in which his phone fires a Pipis that explodes in his face and covers it in soot, rather than launching a volley of flying heads at him. Spamton NEO's head remains covered in soot until his next attack. It's likely that this event was changed due to Spoiler:its accidental resemblance to blackface, which historically was often paired with similar explosion gags in animation.
Contributed by game4brains
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In the Japanese version of the game, the Spoiler:mock-VCR HUD icons that appear during Spamton NEO's ultimate attack in Chapter 2's Weird Route include two kanji-related puns: Spoiler:the "WRECK" icon and the "PAUSE" icon are respectively given the non-standard translations "殺エイ中" and "テイ死" instead of the typical "撮影中" and "ていし". The former includes an alternate reading of the kanji for "kill," while the latter uses the kanji for "death," tying in with Spoiler:Spamton's habit of replacing instances of "だい" and "です" with the phonetically-similar English words "die" and "death" throughout the Japanese script.
Contributed by game4brains
The Tasque enemies' quotes during battle are written in binary, with "me" representing a 1 and "ow" representing a 0. Translating most of these quotes gives each letter in the word "meow" (written in lowercase); meanwhile, the longer meow that Tasques utter after using the "Pet" ACT on them translates to a ♥ symbol.
Contributed by game4brains
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The Version 1.07 patch adds in a new Easter egg surrounding Spoiler:the Pipis enemies that can be encountered on the Weird Route; entering a battle with one now has a 1% chance of spawning "Ms. Pipis", a Pipis with a pink bow on it. This difference is purely cosmetic, and the enemy functions identically to a standard Pipis. In the official notes for the patch, this Easter egg is alluded to by the text Spoiler:Pipis Fanservice, which is tucked in the bottom-right corner of the changelog.
Contributed by game4brains
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Within the data for Chapter 2 is an unused attack for Spoiler:Spamton NEO in which the player is limited to shifting the SOUL between three horizontal planes while dodging oncoming oversized bullets. This attack functions identically to the Purple SOUL mechanic in Undertale, implying that at one point, Spoiler:the Spamton NEO fight was intended to be based around that rather than the Yellow SOUL mechanic.
Contributed by game4brains
Within the data for Chapter 1 is an unused attack for Spoiler:Jevil that consists of multiple spades converging into the center of the Bullet Board in a cross pattern. This attack would eventually be repurposed for the rematch against Berdly in Cyber City during Chapter 2, albeit with tornados in place of spades.
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During the battle tutorial in Chapter 1, if the player repeatedly selects "FIGHT" and misses Kris's attack, Ralsei will grow increasingly agitated, eventually abandoning the tutorial altogether after ten misses and blaming himself for being a poor instructor.

Additionally, the third time Kris's attack misses, Ralsei's response will contain a typo in it, featuring an 8 mistakenly typed to the left of the asterisk that usually starts a line of in-game text.
Contributed by game4brains
In a livestream of Chapter 1 conducted for Undertale's sixth anniversary, Toby Fox stated that the chapter was originally meant to include a number of branching options like in Undertale, such as Kris being able to partner with either Susie or Ralsei depending on their actions up to the first K. Round fight. However, much of these ideas were cut both to avoid dragging out development and to streamline the plot; the resulting linearity ended up being incorporated into the game's narrative, resulting in Chapter 1's heavy focus on a lack of agency.
Contributed by game4brains
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In typical gameplay, Spoiler:the UFO room in Queen's mansion is locked off during the Weird Route in Chapter 2. However, it still remains in the game's code, and modifying the game to access it reveals that Spoiler:the UFO hazards are replaced with the plug hazards seen both near the start of the Cyber Field and near the end of the mansion.
Contributed by game4brains
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Normally, Spoiler:the SnowGrave spell on the Weird Route is only meant to be used against Berdly, who is frozen in a giant block of ice. However, using it against a regular enemy causes them to dissolve into a red mist, similarly to how monsters die in Undertale. This can be seen legitimately by Spoiler:progressing through enough of the Weird Route for Noelle to learn SnowGrave, then sparing Berdly instead of freezing him. Backtracking through Cyber City allows the player to access the cheese maze, where, if they hadn't done so already, they can spawn Maus enemies that can be fought and killed with SnowGrave.

Additionally, this animation was also in the code for Chapter 1, but went unused there since all enemies in the Chapter simply flee from battle if defeated with violence.
Contributed by game4brains
While fighting Spamton and Spoiler:Spamton NEO, the player is able to press F1 to summon a miniature Spamton angel that heals the party, which can only be done once. On the PS4 and Switch versions, this function is normally mapped to clicking the right stick, but connecting a USB keyboard to the console allows the player to activate it with the F1 key anyways.
Contributed by game4brains
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Spamton draws a number of parallels to Mettaton from Undertale throughout his character arc in Chapter 2:

• Both characters have names ending in "-ton", and both Spoiler:have NEO forms that can be optionally encountered. Speaking with Swatch after Spoiler:defeating Spamton NEO on the normal route implies that Spoiler:the robot that became Spamton NEO's body was designed by Deltarune's equivalent of Mettaton.
• Both characters speak in all-caps with idiosyncratic speech patterns (staggered text scrolling for Mettaton, random bracketed text and typos for Spamton).
• The purchasable Spamton-shaped mannequin in Cyber City wears the same dress Mettaton does during the latter's opera performance.
• Before fighting him on the Neutral and Pacifist routes in Undertale, Mettaton makes a statement that appears to align with the protagonist's goals before quickly responding with "NOT!!!" Spoiler:On the Weird Route in Deltarune, Spamton NEO makes a similar jab, even using the exact same formatting for the "NOT!!!" response.
Spoiler:Both Spamton NEO and Mettaton utilize similar mechanics in their fights. Both are fought using the Yellow SOUL, both include miniature versions of themselves with their own attacks, both use identical cross-exploding bombs, both feature attacks where the player must shoot their hearts (which fire back in turn), and both fights involve a free turn for the player (Mettaton EX taking a break, Spamton NEO being attacked by his own phone).
Spoiler:Spamton NEO's "Check" description reads "He's his own worst invention." This nods back to Mettaton NEO's "Check" description, "Dr. Alphys's greatest invention."
Spoiler:One line of flavor text in Spamton NEO's fight reads "The stage lights are shattered," nodding back to flavor text in Mettaton NEO's fight that reads "Stage lights are blaring."
Spoiler:Spamton NEO's battle theme, "BIG SHOT", samples Mettaton NEO's battle theme, "Power of 'NEO'".
Spoiler:Spamton NEO's ultimate attack on the Weird Route includes mock VCR HUD icons that read "WRECK" and "SPEW", nodding back to Mettaton EX's "REC"/"REW" attack.
Contributed by game4brains
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During the epilogue of Chapter 2, the player can encounter the Deltarune counterparts of Undertale's guard dogs as prisoners in the police station's jail, describing themselves as a criminal group called the Wet-Nose Bandits before describing how they were attacked by an unknown individual in the shadows during an attempted burglary over the holidays. The name and backstory of the group reference the Wet Bandits, the main antagonists of the 1990 comedy film Home Alone.
Contributed by game4brains
Spamton has unused dialogue for Spoiler:if the player doesn't have enough Dark Dollars to buy the ThornRing from him on the Weird Route in Chapter 2, responding to their predicament with "[Money] NO". In normal gameplay, Spoiler:the player is always guaranteed to have at least $1997 at this point, making this text impossible to see without hacking.
Contributed by game4brains
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The carton of almond milk that both K. Round and Sweet Cap'n Cakes use to negate damage (as part of their fights' enforcements of pacifist methods) is a compressed stock photo from Alarmy depicting an actual carton of Silk-brand almond milk, editing the logo to simply read "Milk".
Contributed by game4brains
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According to an interview with Toby Fox conducted by Nintendo of Japan, Sweet, Cap'n, and K_K were originally meant to be a single character. However, when concept artist Nelnal sent in three potential designs, Fox ended up enjoying them all so much that he reworked the character into a trio.
Contributed by game4brains
In Chapter 2, two of the in-battle quotes the Virovirokun enemy can utter are "Happy new year 1997!" and "I've got a love letter for you." These are references to two infamous real-world computer worms: Happy99, which was one of the first computer viruses to be widely and rapidly disseminated by email, and ILOVEYOU, one of the most destructive computer viruses of all time.
Contributed by game4brains
If Kris is downed during the Sweet Cap'n Cakes fight before using the "Dance" ACT on each member of the trio at least once, the cutscene where Susie and Ralsei learn the S-Action and R-Action abilities plays prematurely, effectively allowing the player to complete the fight much quicker than intended. The dialogue in the cutscene is also altered to reflect this change in circumstance.
Contributed by game4brains
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Within the game's data is an unused ACT associated with Spoiler:the Spamton NEO fight in Chapter 2, in which Susie throws Kris at Spoiler:Spamton NEO's wires to snap them, similarly to the fights against the Werewires and Spoiler:Queen & Berdly; this ACT even contains unique graphics not seen anywhere else in the game. In normal gameplay, Spoiler:Spamton NEO's wires are instead snapped automatically by selecting the "Snap" and "SnapAll" ACTs, without the need for action commands.
Contributed by game4brains
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In Chapter 2, after Spoiler:Lancer petrifies from being incompatible with the Cyber World, the player can push the resulting statue left and right; pushing Spoiler:Lancer all the way over to the table on the left side of the room results in a plate of spaghetti and a napkin falling onto the table and on Spoiler:Lancer, who will bring up the message "(He seems to be well-taken care of)" when checked.
Contributed by game4brains
In the initial release of Chapter 1, the player could obtain a "Wrist Protector" item at the start of the Dark World if they manage to reach the area in under six minutes; the item allows the player to skip text by holding down the C key.

In the Chapter 1 & 2 release, the Wrist Protector is removed, and its functionality is permanently enabled from the outset, even before entering the Dark World in Chapter 1.
Contributed by game4brains
Taking the mechanical hum outside Hometown's bunker and speeding it up by 666% reveals that it's actually a heavily slowed-down version of "mus_smile", the audio track that plays when Spoiler:accessing the hidden Wingdings version of Entry Number Seventeen in Undertale.
Contributed by game4brains
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The characters Ruddin, Hathy, Clover, Lancer, and King all took their designs from a series of playing card illustrations that Tumblr user Kantonytes published in 2012, the year Deltarune first started development. In a tweet posted shortly after Chapter 1's release, Toby Fox credited Kantonytes' work as a key inspiration for the game as a whole.
Contributed by game4brains
If the player hacks the game to make Kris equip the normally functionless BrokenSword item, they will gain the "Failure" ability, which is unique to this weapon (though it lacks any special effects).
Contributed by game4brains
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Chapter 2 contains unused data for Susie gaining a plushie of herself in her room should Kris give her Queen's gift during the party's reunion at the Cyber City carnival. Adding the Susie plushie back in and interacting with it results in unique dialogue between Susie and Ralsei commenting on it.

In a tweet posted off the heels of Chapter 2's release, graphic designer Temmie Chang showcased not only the sprite for the Susie plushie, but also sprites for a Ralsei plushie and a Noelle plushie, indicating that Queen's gift was originally planned to morph into the likeness of whoever the player decides to give it to. In normal gameplay, the only selectable character who opens the gift is Berdly, who finds a plushie of himself with nipples.
Contributed by game4brains
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In the initial "SURVEY_PROGRAM" release of Chapter 1, the Librarby's computer lab entrance uses a single door; Chapter 2 however changes this to a set of double doors, with the accompanying sign being horizontally squished and moved to the left side of the doorframe. Several stray pixels in the Librarby interior are also cleaned off.

Because Chapter 2 takes place just one day after Chapter 1, the Chapter 1 & 2 release changes the Librarby interior in Chapter 1 to match that of Chapter 2.
Contributed by game4brains
Within the files for Chapter 1 is an usused dialogue chain that simply reads "Blaze it." before opening a prompt where the player must choose between "Blaze" and "Do Not Blaze"; both options do nothing. The dialogue has an ID number of 420, which combined with its contents references the famous stoner catchphrase "420 Blaze It."
Contributed by game4brains
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Within the files for Chapter 1 is unused battle data, including graphics and text, for a fight against Ralsei. As the unused text includes mentions of the training dummy seen in the final game, this data suggests that the game's combat tutorial would've originally seen Ralsei partner with the dummy instead of with Kris.
Contributed by game4brains
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During the vessel creation sequence at the start of Chapter 1, if the player gives their creation and themselves the same name, the in-game narration will respond with a passive-aggressive comment about how on-the nose that is.
Contributed by game4brains
Within the data for Chapter 2 are an unused series of voice grunts; the filenames for them are all "Snd_txtrx" followed by a number. This name deciphers as "Sound Text Rx," implying that these files were intended for Rouxls Kaard, who during regular gameplay is one of the only characters to feature dialogue portraits but no voice grunts (instead using the generic text scrolling sound).
Contributed by game4brains
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If the player Spoiler:downloads Spamton onto the EmptyDisk and attempts to use the resultant Spoiler:LoadedDisk item on the purchasable mannequin in Cyber City, a text box will appear stating that the item "squirmed and won't fit." This Easter egg appears to foreshadow Spoiler:the actual purpose of the LoadedDisk, which Spamton uses to become Spamton NEO by uploading himself to the discarded robot in the mansion basement.
Contributed by game4brains
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While exploring Cyber City with Noelle in Chapter 2, the player can encounter a hidden alleyway marked with dog signs. Going down this path results in Kris being run over and downed by the Annoying Dog, only for the Undertale game over screen to appear in place of the standard one. The Annoying Dog proceeds to run over that too, after which Kris and Noelle wake up beside a dumpster with a Dog Dollar added to the player's inventory.

This Easter egg nods back to a Tweet posted by Toby Fox in 2019, which consisted solely of the Annoying Dog's car sprite with the caption "this is the last thing you see before you die."
Contributed by game4brains
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Ambyu-Lance's recruit bio ends with the line "It's not down with the sickness." This plays off the title of the 2000 song "Down with the Sickness" by American nu-metal band Disturbed.
Contributed by game4brains
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During the Spoiler:Giga Queen fight at the end of Chapter 2, if the player manages to avoid taking any damage during rounds one and two, Spoiler:Giga Queen will become enveloped with a blue aura during round three, and her attacks will become more aggressive and harder to avoid. To compensate, however, her defense is reduced.
Contributed by game4brains
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In Chapter 2, if the player activates the Warp Door in Queen's mansion and uses it to revisit Cyber City, they will find one of the paths blocked off by an oversized toilet sprite. According to a tweet posted by Toby Fox during Chapter 2's development, the sprite was created at his request by the game's primary graphic designer Temmie Chang, only to end up absurdly large due to a miscommunication.

Furthermore, interacting with the toilet produces the message "(For some strange reason, a giant toilet-shaped toilet is blocking the way.)" This is a reference to the Iron Pencil statues in EarthBound, which produce the near-identical message "(For some weird reason, a pencil-shaped iron statue is blocking the path.)" when checked.
Contributed by game4brains
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While Ralsei's manual in Chapter 1 is not readable in normal gameplay, unused graphics exist for it in the game's data. This readable version of the manual appears to have been taken out at an earlier point in development, as some of the information included is markedly different from actual gameplay. For instance, the manual describes unique action keys for each character during fighting (the final game uses Z universally), lists an overworld menu button for talking to party members, and describes TP as conserved between battles and restored at save points like HP (the final game converts leftover TP to Dark Dollars after battles).
Contributed by game4brains
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Within the game's code is an unused attack for Spoiler:the first fight against Queen in Chapter 2 that summons bouncing trails of spade cards; this attack references the 1990 version of Windows Solitaire, which features a similar "bouncing cards" effect whenever the player wins.
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In one room of Spoiler:the mansion basement in Chapter 2, the player has a rare chance of seeing a grinning face appear out of bounds at the far left before quickly fading away. The face's pink and yellow eyes match those of Spamton, who by this point Spoiler:has instructed Kris to retrieve the EmptyDisk from the basement.
Contributed by game4brains
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During the spotlight puzzle segments in Queen's mansion, if the player moves the bird-shaped light onto Berdly and keeps it there for long enough, Berdly will claim that Kris is trying to taunt him, before claiming that they're only able to do so because "this isn't online." This is a reference to Super Smash Bros. Ultimate, which disables the use of taunts during Quickplay matches.
Contributed by game4brains
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Chapter 2 contains an unused alternate set of battle sprites for Susie depicting her with her eyes exposed; while her eyes are visible on her overworld sprites, standard gameplay uses the same battle sprites for Susie as Chapter 1, which depict her with her bangs covering her eyes.
Contributed by game4brains
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In the initial release of Chapter 2, interacting with Undyne's desk after goading her into breaking it generates the placeholder text "[NO TEXT] (obj_npc_room)"; a subsequent patch resolved the issue by making the desk impossible to interact with after it's broken.
Contributed by game4brains
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In the Japanese version of Chapter 2, the cell Queen places Kris in (which is decorated with items based on their internet search history) includes only a single Toriel hologram rather than two. This is due to the fact that the pun involved with the pair (Kris mis-typing "video game piano tutoriel") can't be easily conveyed in Japanese.
Contributed by game4brains
If the player builds the duck version of the Thrash Machine in Chapter 1 and imports the resultant save file to Chapter 2, its attacks during the Spoiler:Rouxls Kaard fight become extremely easy to avoid; Spoiler:Rouxls and Ralsei will also have unique dialogue surrounding the machine's design.

Later, when the Thrash Machine is incorporated into the mech used to fight Spoiler:Giga Queen, the duck version replaces the standard punch sound effects with squeaking noises and features "DuckMode" as its offensive ACT, which reduces the move's strength in exchange for slightly draining additional health from Spoiler:Giga Queen between turns.
Contributed by game4brains
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Within the data for Chapters 1 and 2 is an unused weapon item called the Trefoil; hacking it into a Chapter 2 save file and attempting to equip it to Noelle results in her saying "Okay! ...? What do you mean, unused!?"
Contributed by game4brains
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If the player talks to Spamton in his shop and selects the "OUR DEAL" option after purchasing the Keygen item from him, Spamton will simply say "KEYGEN" and oscillate between various colors while the dramatic chiptune track "KEYGEN" plays. This is a reference to "The King Downloads Sony Vegas", a 2009 YouTube Poop by Moto200Alt, which depicts King Harkinian (from Link: The Faces of Evil and Zelda: The Wand of Gamelon) flashing between a negative and positive image of himself after opening an illicit keygen for Sony Vegas and being awed by its chiptune soundtrack.
Contributed by game4brains
During the fight against Spoiler:Spamton NEO in Chapter 2, a sped-up voice can be heard at the end of his battle theme, "BIG SHOT", muttering phrases such as "they've come for me" and "please pick up the phone" before laughing. On the official soundtrack album, however, this voice is absent; it is unknown whether this is a deliberate omission or an oversight.
Contributed by game4brains
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Ruddin's recruit bio ends with the line "Shine on you lazy diamond!" This plays off the title of the 1975 song "Shine On You Crazy Diamond" by British progressive rock band Pink Floyd.
Contributed by game4brains
According to Toby Fox, Susie was originally inspired by Maya Fey from the Ace Attorney series, being a similar "small, friendly, sassy companion" without any major tension. However, she was eventually changed into a more acrid figure as development progressed.
Contributed by game4brains
According to creator and director Toby Fox, the idea for Deltarune originated in 2011, when he had a literal fever dream about witnessing the ending to a game that didn't exist; upon waking up, he became determined to make that game a reality.

Development originally started the following year, but was put on hold before designing even a single room before the project resumed after Undertale's release; two of the songs originally composed during that initial stage would later be reused in Undertale as "Bonetrousle" (previously the main battle theme before being replaced with "Rude Buster") and "Heartache" (previously titled "Joker Battle", apparently being Spoiler:Jevil's early battle theme before being replaced with "THE WORLD REVOLVING").
Contributed by game4brains
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During the Spoiler:Spamton NEO fight in Chapter 2, the player can exploit a glitch that forces Spoiler:the yellow SOUL to fire multiple charged shots in quick succession with the Z and Enter keys, pressing one while holding down the other.

A subsequent patch chose not to remove the glitch, but rather introduce a penalizing Easter egg based on it, where Spoiler:firing six to ten charged shots with this exploit angers Spamton NEO and results in his attacks scaling up in damage with each successive use of the glitch.
Contributed by game4brains
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The game's files contain an early version of the computer lab seen at the end of the game, with a calculator similar in design to Queen in place of a public laptop. As the Cyber World was formed from a Dark Fountain opened in the computer lab, this implies that Queen was originally meant to be themed around calculators rather than computers, as indicated by the red text that sometimes appears on her visor.
Contributed by game4brains
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Within the files for Chapter 2 is a fully-written recruit bio for Spoiler:Jevil, the hidden boss of Chapter 1; this cannot normally be accessed even if the player pacifies him in Chapter 1 and imports the resulting save file to Chapter 2.

Curiously, although recruiting enemies requires sparing or pacifying them, Spoiler:Jevil's bio mentions the Devilsknife item earned from defeating him violently, rather than the Jevilstail item earned from pacifying him.
Contributed by game4brains
During the opening sequence where the player creates a "vessel," attempting to name their creation "Gaster" will instantly reset the game back to the title screen. Attempting to name yourself "Gaster," meanwhile, will cause the game to forcibly quit. This resembles an earlier Easter egg found in Undertale, where attempting to name the Fallen Child "Gaster" will similarly trigger a reset.
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If the player backtracks through the Starwalker's room in the Forest after ringing the bell to scare them off, they can encounter an NPC called "The Original Starwalker." Speaking to this character causes them to reappear in the castle basement's animal cage room.

Importing a Chapter 1 save file to Chapter 2 after discovering the Original Starwalker results in the latter entering Kris' key items alongside Lancer and Rouxls Kaard, briefly exiting to accompany them at the carnival and appearing as part of the Thrash Machine's transformation scene.
Contributed by game4brains