When solving Cyber Field's typing puzzles in Chapter 2, stepping on an A key will, on rare occasions, play Sans' voice grunt instead of the usual text-to-speech clip.

The dance that the miniature Sweet Cap'n Cakes doodles perform both when attacking and during their post-battle cutscene references "Danjo", a popular 2008 Vocaloid fan animation set to the 2006 song of the same name by Japanese artist Tarou.

The track "Honksong.ogg", which plays during the traffic jam that prevents Kris from going past the Librarby in Chapter 2's prologue, interpolates excerpts from the Undertale tracks "Your Best Friend", "Spear of Justice", "Dogsong", and "Metal Crusher".

After reaching the lobby of Queen's mansion in Chapter 2, if the player backtracks all the way back to and interacts with the very first save point in Cyber Field, the accompanying flavor text will change, noting how the player has already seen this area before riffing on how their backtracking served no real purpose whatsoever.

The intro to the track "WELCOME TO THE CITY" interpolates the chorus melody to "74", a 2018 song performed by J-pop singer Itoki Hana that Toby Fox had previously composed and provided guest vocals for.

Within the data for Chapter 2 is an alternate, sped-up version of "Digital Roots" that lacks the ambient wind sounds. The existence of this track is a marked deviation from the rest of both Deltarune and Undertale, which otherwise speed up and slow down tracks in real time.

The filename for this unused variation is "spamton_house.ogg"; as "Digital Roots" itself uses the filename "spamton_basement.ogg", this seems to imply that the faster version was originally meant for Spamton's shop, which instead uses "Dialtone" (named Spoiler:"spamton_neo_after.ogg" in the game's files) in the final game.

The CHECK description for Spoiler:the Pipis enemies, which can only be encountered on the Weird Route, reads "Pipis - ''The Original'' An invasive species of freshwater clam." This appears to reference two real-world entities that Spoiler:are also called Pipis. The first portion of the description seems to nod to a restaurant in Puerto Vallarta, Mexico, which uses the tagline "The Original." The second portion, meanwhile, references a pair of edible saltwater clam species in Australia and New Zealand; the use of these clams as food is further referenced with Spoiler:Kris' "Fried Pipis" ACT during the Spamton NEO fight at the end of the Weird Route.

Within the data for Chapter 2 are unused animations for both Lancer and Rouxls Kaard where they transform into their respective Light World forms (a Jack of Spades card and a rules card) when entering the player's inventory. In typical gameplay, the associated code for these animations is deactivated, with Lancer & Rouxls simply sliding into Kris' sprite.

In the original release of Chapter 2, Spoiler:the "WRECK", "PAUSE", and "SPEW" VCR icons that appear during Spamton NEO's ultimate attack only showed up during the hidden Weird Route, and thus were absent from his fight during the normal route. While this difference was initially interpreted by fans as an intentional artistic decision, in actuality it was the result of an oversight: Spoiler:the icons were coded to appear at certain coordinates relative to the room where Spamton NEO is encountered. Because the fight on the normal route moved Spamton NEO and the player far out of range, the icons were consequently left offscreen.

This error was eventually fixed in the v1.08 patch, with Spoiler:the icons appearing in the proper location on both routes instead of just one. The patch additionally adds a new smoke effect after Spoiler:Spamton NEO's ultimate attack ends on both routes.

In the Undertale sixth anniversary livestream, Toby Fox stated that the scene in Chapter 1 where Susie holds off from attacking Kris due to her respect for Toriel was inspired by a real encounter with a bully that he had. According to Fox, the bully knew his mother and stated that they would've beaten him up if she wasn't nice to them.

After Queen realizes that she didn't capture Ralsei out of all of the Lightners due to not having enough cages, the sound effect that plays with the Iris Out zooming in on her face is the same sound effect used on the Iris Out on Mario when completing a level in Super Mario World.

In Volume 5 of his Famitsu column "Toby's Secret Base", creator and director Toby Fox revealed that Ralsei's name came from a period in elementary school where he and his brothers constantly experimented with RPG Maker, having long had a shared interest in game development. Fox's oldest brother spent years working on a game called New Genesis, which featured a protagonist named Ralse; Fox simply appended an "i" to the name when incorporating it into Deltarune years later.

In the same column, Fox stated that the prolonged development of New Genesis discouraged him from making games for a long time. When he finally returned to the field in his teenage years, he sought to temper his expectations from the outset by making smaller-scale titles and preemptively planning out his approach to development to avoid biting off more than he could chew.

In 2012, Toby Fox posted several Yume Nikki fan songs to his Tumblr account. One of these, "waltz of seccom masada" (named after a fan nickname for the piano-playing NPC in the spaceship area), bears strong similarities to the Deltarune track "Man" (heard in the hidden rooms where Kris can obtain an egg from an invisible NPC behind a tree), indicating that Fox repurposed and rearranged the song for his own project years later.

Although Chapter 1 was given a surprise release in 2018, with Toby Fox giving no prior public indications of its existence, Deltarune had been teased in secret since at least 2015. Three months after Undertale released, the fan site DeltaRune.com announced that it would be rebranding to Dreemurr.com at Fox's request; Fox didn't tell the owners of Dreemurr.com why he didn't want them to use the domain. However, after the fan site fulfilled his request, Fox used deltarune.com to host an image called him.png, a passage of Wingdings text only readable by turning up the brightness using image editing software.

While the text initially parroted the phrase "THIS NEXT (space) EXPERIMENT (space) SEEMS (space) VERY (space) VERY (space) INTERESTING" from the unused room_gaster event in Undertale, by July 1, 2016, the text was edited to read "THREE HEROES APPEARED (space) AT WORLD'S EDGE", later being edited again by August 17 to say "THREE HEROES APPEARED (space) TO BANISH THE ANGELS HEAVEN". Both revisions quote portions of the legend that Ralsei recounts in Chapter 1, indicating that these parts of the game's backstory were already conceived by this point.

Following the release of Chapter 1, deltarune.com would be refurbished as the official website for Deltarune itself. Consequently, earlier versions of the site only survive through snapshots on the Wayback Machine, a URL archiving platform hosted by the Internet Archive.

In an interview that Toby Fox conducted with the Chapter 2 development team, Taxiderby, one of the game's programmers, stated that the Spamton cherub that appears when pressing F1 during the Spamton and Spoiler:Spamton NEO fights was a spur-of-the-moment suggestion. According to them, Spamton's line "[Press F1 For] HELP" was already written, but it was originally a meaningless non-sequitur. After Fox suggested the idea of giving F1 an actual function in the fight, Taxiderby came up with the Final Fantasy-inspired Easter egg, putting it in singlehandedly after Fox gave his approval for the idea. In the same interview, they stated that they were surprised by its popularity, having made it purely as a throwaway gag.

Within the data for Chapter 2 is a placeholder graphic for Queen's arcade cabinet that depicts it with green paneling and the word "Xbox" crudely written on the marquee. Despite the fact that it was never intended to be featured in the normal course of play, the sprite was altered in the version 1.06 update to remove the "XBOX" text, presumably to avoid potential trademark issues.

Within the data for Chapter 4 are unused sprites depicting Susie playing a handheld video game while hanging out with Noelle. Concept art for this was shown off in a tweet by Toby Fox in 2022, alongside artwork of two other character interactions. Notably, the Susie and Noelle scenario is the only one shown in the tweet that isn't featured in the final game.

In Spoiler:the Weird Route version of Chapter 4, one pivotal scene depicts Spoiler:Kris, possessed by the SOUL, forcing Noelle to re-don the Thorn Ring from Chapter 2 by slowly creeping towards her as she backs away in fear across a couch. In the original release of the chapter, this gives way to an animation Spoiler:of a rose blooming and then crumbling against a black screen, located in the same place as Noelle's sprite. In the Version 1.01A update, this was replaced by Spoiler:a single red pixel, possibly as a placeholder graphic, which in turn was replaced in Version 1.01C with another animation depicting cracking red glass. The original animation is outright removed from the game's files in these later updates rather than being merely disabled.

The reason for this alteration was not explained in the official changelogs, which respectively described the changes as a "Small graphical alteration to a certain scene" and "Strengthened imagery while maintaining a scene's intended meaning". The original animation was likely changed in response to complaints from players who interpreted it as Spoiler:Noelle being forcibly "deflowered" (taking her virginity, in this case through rape) literally, in addition to being driven insane, and to counter-complaints Spoiler:lamenting the reduced emotional impact when the single red pixel was implemented. Spoiler:The couch sequence moments prior remained unchanged with each update, making it unknown if Spoiler:the rape implication was intended, presumably metaphorically, in all versions of the scene.

When looking into one of the grocery baskets filled with fruit in Sans' grocery store, it states that it contains "(Grapes, melons, oranges, and the scent of fresh pineapples...)". This references the DK Rap from Donkey Kong 64, specifically the outro line "walnuts, peanuts, pineapples smells, grapes, melons, oranges, and coconut shells".

In Chapter 2, when Queen challenges Kris to an arcade match, the arcade cabinet presented is so large that Kris is unable to reach the controls. While the released game conveys this solely through narration, its internal data contains an unused animation depicting Kris reaching up towards the cabinet's joysticks. It is unknown why this was removed, given that it fulfills the same function as the textual description.