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.

In the September 14, 2023 edition of the Undertale/Deltarune newsletter, Toby Fox revealed that a stealth sequence was planned for Chapter 3, only to be cut partway through development due to his dissatisfaction with it, stating that "a mechanic which basically just makes you move slower isn't necessarily fun." To compensate for the segment's removal, Fox included an MP3 file of the music that was composed for it, as the song would not be featured in the final game.

The emblem on Ralsei's robes went through two different renditions in early development. Initial concept art by Toby Fox depicts him with a fireball icon on his robe, matching a statement in an interview where he stated that "[he] was going to give a certain character a fire spell, but decided against it for the first chapter." Later pre-release concept art by Temmie Chang, meanwhile, depicts Ralsei's robes as bearing an upside-down spade symbol. Come the release of Chapter 1, and the design would eventually be changed to a heart; however, most of Ralsei's overworld sprites in Chapter 2 change this to the inverted spade design.

Rouxls Kaard's line "Lost...? Frightened...? Confused...? GOOD!! HA HA HA!!!" appears to be a reference to the 1995 edutainment title I.M. Meen, in which the eponymous antagonist says a similar line.

In the livestreamed playthrough of Chapter 1 conducted for Undertale's sixth anniversary, it was revealed that Lancer's childlike characterization was originally far more subdued. Toby Fox later chose to exaggerate these elements after playtesters instead assumed that Lancer was a Danny DeVito-esque adult.