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).

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.

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.

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.

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.

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.

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.

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.

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!?"

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.

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.

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.

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.

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").

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.

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.

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.

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.

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.