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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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".

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.