In Chapter 4, the piano in the first sanctuary allows for a much more expansive range of notes than other in-game pianos. Accordingly, Susie and Ralsei have special reactions if the player inputs specific songs on it:

• Playing "Memory" results in Ralsei complementing it by comparing it to a lullaby.
• Playing "Your Best Friend" from Undertale results in Ralsei calling it "friendly and sweet," alluding to Flowey's introductory cutscene in that game.
• Playing "The Legend" flatters Ralsei, to which Susie responds by joking that Kris is flirting with him.
• Playing "Lost Girl" results in Susie wondering if she could learn it on guitar.
• Playing "Rouxls Kaard" results in Ralsei recognizing it as Rouxls' theme song; Susie then jokingly calls it the "theme of him getting his ass kicked."
• Playing "Heartache" (Toriel's boss theme in Undertale) causes Ralsei to describe it as "emotional" before Susie suggests playing it for Toriel.
• Playing "Undertale" results in Susie deriding the song as "crap."
• Playing "Dark Sanctuary" results in Susie pointing out that "Ralsei was humming it earlier."
• Playing the song on the Old Man's sheet music results in different responses depending on whether or not the player includes the octave switch: omitting it results in Susie & Ralsei commenting on its absence, while including it results in Susie commenting on how different it is.
• Playing the first four notes of "MEGALOVANIA" (Sans' boss theme from Undertale) results in the Annoying Dog abruptly running Kris over with the car from Cyber City's hidden alley in Chapter 2, which causes a smash to black before the game returns to normal. Attempting to replay the song results in Kris fumbling the fourth note.

Several other songs that produce reactions are transcribed unusually, and as such are more difficult to trigger if playing solely by ear:

• Playing A, D, E, F, D, D, C, D, B, A, D, E, D, C, D, D causes the pair to comment on the "annoying" nature of the melody.
• Playing "Don't Forget" (C, D, E, E, E, E, E, A, C, B, B, B, B, B, B, C, B, A, A, A, A, A, A, G, H, G, E, C) causes Susie to fall silent before calling the song "kinda cool."
• Playing "Spear of Justice" (E, D, B, C, A, B) results in Susie calling the song "majestic" before Ralsei suggests that the Old Man would enjoy it, referencing the song's nature as Spoiler:both Undyne and Gerson's leitmotif.
• Playing "Chopsticks" by Euphemia Allan, "Heart and Soul" by Frank Loesser & Hoagy Carmichael, or "You've Got a Friend in Me" by Randy Newman & Lyle Lovett (C, E, C, F, E, A, F, H, F, H, E) results in Susie scolding Kris for "messing around." Notably, all three of these are real-life, commercially released songs: the first two are common stock piano pieces, while the latter was recorded for the 1995 animated film Toy Story. Its inclusion likely references a series of comedic edits of the song posted to Tumblr by developer Toby Fox. One of these edits, "Friend inside Me", was previously the subject of tongue-in-cheek fan theories claiming that it would pertain to Chapter 3's hidden boss.

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.

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.

After Kris exits the bathroom during Chapter 2's epilogue, the interior of the Dreemurr household noticeably changes: the phone and the painting by the trash can are both moved further up the wall, and the refrigerator grows noticeably taller.

These changes are also present in a mockup of the interior found in the game's files. However, the mockup is set during the day instead of at night, and the painting beside Toriel's chair is moved upward. This implies that the nighttime version of the interior was based on the mockup (with only minor touch-ups) without cross-referencing the layout seen earlier in the game.

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.

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.

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.

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.

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 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 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.

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 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".

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.

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.

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.

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.

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.

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.