Many lines of battle dialogue associated with the Shinobeetle enemy reference the English rock band The Beatles:

• "The sound of reeds, helter and skelter." references the band's 1968 song "Helter Skelter".
• "Blackbirds, calling to the dead of night." is a modification of the opening lyric to the band's 1968 song "Blackbird".
• "Rice, where a wedding had been." alludes to the line "picks up the rice in a church where a wedding had been" from the band's 1966 song "Eleanor Rigby".
• "Love me... DO!!!!" quotes the title of the band's 1962 debut single.
• "Phew, it's just me? Time to think for yourself!" references the band's 1965 song "Think for Yourself".
• "Peace and love!" quotes a catchphrase associated with drummer Ringo Starr.
• "Could I get a little help from my friends?" references the band's 1967 song "With a Little Help from My Friends".
• "I'm happy just to dance with you!" quotes the title of a track from the band's 1964 album A Hard Day's Night.
• One of Ralsei's R-actions reads "Ralsei listens to a white album!", referencing a popular nickname for the band's 1968 self-titled LP, coined on account of its all-white cover.

Within the data for Chapter 5 is an unused mechanic for the Flowery fight where, upon selecting a "Flower" option in the battle menu, the player would use the Orange SOUL to ram into one of five different ACTs in phases 2, 3, and 4, with the correct ACT corresponding to that phase's group ACT in the final game. Additionally, failing a given phase's puzzle would increase the mercy gained by using SpinZ during phase 2 (Seth & Aqua) and PraiseZ during phase 3 (Green & Orange). The code for the fight also includes an unused mechanic where Spin/SpinZ and Praise/PraiseZ are boosted to 80% Mercy. The Spin/SpinZ buff would activate if the player defeated Seth & Aqua violently at the cliffs, while the Praise/PraiseZ buff would activate if the player defeated the chapter's secret boss. The latter would also activate an Easter egg where Spoiler:Pink shows up during the ACT and flashes a peace sign while laughing.

Within the data for Chapter 5 is an unused hint system for the trial mechanic during Blue & Yellow's fight. By selecting a "Hint" ACT on Blue, the latter would provide a brief explanation of that phase's evidence that alludes to the true culprit. The game's data also includes variables for Justice and Integrity, implying that using the act on Yellow would provide a different set of hints written around his own characterization. The full set of Blue's English-language dialogue for this act is stored in lang_ja.json, indicating that it stayed into the game for long enough to just barely reach the translation phase.

In Chapter 4, if the player leaves the Second Sanctuary's version of Gerson's study after entering it, the party will enter a fight against an Organikk and a Wicabel. The opening flavor text for this fight reads "It is known." This is actually placeholder text meant for use with test enemies, which the game otherwise defaults to if an encounter lacks a unique definition in the game's code. While its presence here is nominally due to this specific encounter not having a defined start-of-battle message, modifying the game to replicate this oversight normally produces a blank text box, making the placeholder's presence even more anomalous.

In Chapter 4, if the player attempts to backtrack out of the room where Spoiler:Susie encounters the final panel of the prophecy and destroys it before Kris can read it, they will be prevented from leaving, with an accompanying text box describing how "another hand pressed against yours and pushed you back." Once this dialogue ends, the game is coded to play snd_guitar_hit_03, one of the sound effects for Susie playing Dess' guitar. However, the sound effect's volume is set to 0%, leaving this detail unnoticeable in normal gameplay.

Within the data for Chapter 5 is unused alternate dialogue for Seth & Aqua's break in Flower Cafe, wherein Aqua asks Seth what their gender identity is. Seth responds by describing it as "Something as cool as a bluebird... That a blue bird... also thinks is the coolest!" An embarrassed Seth then asks Kris to help them explain after Susie and Ralsei react with confused silence. This dialogue is coded to appear if the break is selected without having recruited every flower. However, because recruiting every flower is a requirement for the break to be accessible in the first place, this sequence goes unused.

Within the data for Chapter 4 are two unused prophecy panels pertaining to optional and hidden events in the game's story:

• One panel depicts a series of snowflakes and is accompanied by text reading "COLDWATER FELL FROM THE TOP OF THE SKY." This alludes to Spoiler:the Weird Route, which is instigated in Chapter 2 by using Noelle's IceShock ability to freeze every enemy in Cyber City before learning and using SnowGrave on Berdly.
• The other panel depicts the battle hammer from Spoiler:Gerson's Hammer of Justice guise, with its own text reading "AXE CARVED BY THE TORTOISE'S GRAND HAMMER." This alludes to Spoiler:the battle against Gerson himself as the chapter's secret boss and the JusticeAxe weapon that he grants Susie upon defeat.

Although the hammer panel is never seen as a part of the prophecy, the sprite for it is nonetheless featured among the cavalcade of panels floating in the void at the start of the Third Sanctuary. The snowflake panel is also alluded to in the Winter 2025 edition of the Undertale/Deltarune newsletter, the start of which reads "Cold water falls from the top of the sky."

In Chapter 3, Kris' sliding sprite is replaced with a new, fully animated design depicting their hair waving in the wind as they face downward, aligning with Susie and Ralsei's sliding animations. Chapters 4 and 5 only display the first frame of this animation due to a coding error, in which slopes lack the additional data which sets Kris' animation speed, causing it to default to the lowest possible value (i.e. static). The sole exception is the first sliding wall in the Cliffs during Chapter 5, which features a different error wherein the slope sets Kris' animation speed too high.

As Chapters 3 & 4 were originally released as a launch title for the Nintendo Switch 2, that platform's version of the Mike Zone is based around using both Joy-Con 2s as mice simultaneously. In other versions of the game, this entire sequence is substantially altered. The PC version pairs a single mouse with the computer's microphone, which detects how loudly the player speaks into it. If a microphone isn't connected, the game instead has the player hold down a key to mimic raising one's voice. Console versions also mimic this by using the shoulder button to control the microphone's volume. The Mike fight's mechanics are changed to account for these controls, and the minigames Air Waves and Main Performance are replaced with Battat and Jongler. The sole minigame present in all versions, Pluey, is also reconfigured and given to Cat Mike outside the Switch 2 version rather than Small Mike. As a side-effect of these changes, Green Pippins' dialogue at the end of the Trashy Trio fight in Chapter 5 is also different between platforms. In the Switch 2 release, he scolds the characters for "misusing the AIR WAVES motif," tying in with how both of the songs "Inappropriate Recycling" and "AIRWAVES" feature a recurring leitmotif introduced in "The World Revolving" (colloquially referred to as the "Freedom Motif"). On other platforms, Green Pippins instead refers to it as the "BATTAT motif", as the song "AIRWAVES" is transferred to that minigame in those releases.

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 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." An early version of the backstage area based around this concept is still present in the game's data, while the sneaking mechanic in the final version of Chapter 3 is heavily simplified and faster-paced.

To compensate for the original segment's removal, Fox included an MP3 file of the music that was composed for it, which would eventually be incorporated in Chapter 5 for two optional cutscenes if the player knocks on the Mike Room's door in Castle Town.

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 a retrospective post detailing the making of Chapters 3 and 4, director Toby Fox revealed that the character design of Tenna was closely inspired by Mr. Tv Dinnah from the game Little King's Story, while the unconventional 3D graphics in the game Ghost Trick: Phantom Detective inspired him to use pre-rendered sprites for Tenna. Fox first attempted to shoot motion capture rigs with a Kinect and then import them into the animation program MikuMikuDance. Later, after drawing concept art for the character in 2017, he attempted to have Kanotynes create a 3D model for him, though this fell through for unspecified reasons. The final model for Tenna's sprites was ultimately put together by PixelatedCrown based on Gigi DG's interpretation of Fox's artwork.

In a retrospective article about the development of Chapter 5, developer Toby Fox stated that Flowery was inspired by a common misspelling of Flowey's name among Undertale fans in the months following that game's release. This is reflected in early concept art showing Flowery with a head-and-neck collar designed after Flowey's petals and a green hoodie corresponding to Flowey's stem. Flowery's final design was put together by Studio Khara staffer Touko Yatabe during the production of his introductory FMV.

The Flowey inspiration is further alluded to throughout Flowery's appearances in Chapter 5. In addition to being a golden flower in the Light World, Flowery's initial theme, "Your Dad's Best Friend", nods to the title of Flowey's motif, "Your Best Friend". The latter is itself quoted partway through Flowery's battle theme, "Flower Man". Additionally, during the battle against Flowery, the flavor text will occasionally read "Flowery emits the fragrance of hope!" and "Flowery holds onto his dream.", referencing "Hopes and Dreams", the battle theme against Spoiler:Asriel, Flowey's true form, in Undertale. One of Flowery's lines during the fight additionally muses on a version of events where he and the party didn't have to fight one another, calling it a "pacifist root," referencing the colloquial term for the Undertale route wherein the player beats the game while sparing every monster, culminating in Spoiler:the aforementioned Asriel fight.

Prior to Chapter 5's release, the "Flowery" misspelling was also alluded to in an interview with Sans and Papyrus included in one of the editions of the Undertale/Deltarune newsletter, in which Papyrus repeatedly mis-remembers Flowey's name as "Flowery."

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.

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 "Your Dad's Best Friend" causes the pair to comment on the "annoying" nature of the melody, alluding to Flowery's portrayal in Chapter 5 as obnoxiously heroic and the animus that Ralsei displays towards him.
• 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 "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.