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.

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.

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."

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.

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.

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.

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.

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.

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.

During the fight against Orange and Green, Orange opens the pair's first turn by commanding Green to "rev up those fryers." This appears to reference a line from the 2000 SpongeBob SquarePants episode "Arrgh!", spoken by the recurring minor character Fred as he enters the Krusty Krab. At a later point in the fight, Orange compliments Green's cooking skills by stating that "these nuttrons will have you rustling in your jimmies." The corruption of "nutrients" as "nuttrons", coupled with the word "jimmies", references "Jimmy Neutron Happy Family Happy Hour", a surreal internet animation parodying the 2002 animated series The Adventures of Jimmy Neutron, Boy Genius where the title character is mispronounced as "Jimmy Nuttron". Meanwhile, the phrase "rustling in your jimmies" itself is a reference to the memetic phrase "that really rustled my jimmies".

The allusions to two different cartoons produced and aired by the TV channel Nickelodeon likely alludes to the network's use of orange and green in its visual branding.

In console releases of Chapter 5, when Kris, Susie, and Ralsei ascend the beanstalks leading from the Gardens to the Cliffs, the surrounding border scrolls alongside the level. This includes both the large stalk in the platforming section and the second, smaller stalk during the RPG section.

Within the data for Chapter 3 are unused sprites depicting brown, black, and gray deer; the gray deer acts as a hurt sprite for the black one. The brown and black deer are early versions of Spoiler:the White Cloak and the movement-copying black monster found on the Sword Route respectively. The former NPC's internal filename, "deer", is a remnant of this initial idea. Of note is that the game uses a variable named "toriel" to toggle these characters' sprites between the deer and the designs seen in normal gameplay.

In Chapter 5, if the player loses to the first Floradinn encounter, Kris, Susie, and Ralsei will wake up in three literal flower beds in a unique room in the Garden. Subsequently, Flowery will appear and reveal that he rescued the party after finding them "unconscious, washed up on the beach," expressing concern for their safety and discouraging them from continuing their adventure. Exiting this room places the party in the area where the first RedRibbon is obtained.

In a retrospective article detailing the making of Chapters 3 & 4, developer Toby Fox stated that the sequence where Spoiler:the SOUL travels alone throughout the Holiday Mansion went through several disparate revisions during development to "make you feel like it wasn't wasted." Fox stated that he initially conceived "a Rube Goldberg machine, involving many crazy parts" which the player would need to utilize in order to manipulate character actions. Subsequently, this was replaced with a sequence where the player utilizes a model train set spanning the whole of the house. When this didn't work, he conferred with personal friend and Homestuck author Andrew Hussie, who suggested a puzzle requiring the player to complete a bust parodying Macaulay Culkin's pose on the theatrical poster for the 1990 Christmas comedy film Home Alone.

Ultimately, Fox opted for the final game's approach, in which Spoiler:the SOUL simply navigates the mansion vents to spy on various characters, after realizing that "just focusing on the story and atmosphere is the best." Despite this, remnants of the early ideas remain in the released version of Chapter 4. The tail end of the sequence includes Spoiler:a scene in which the SOUL has to manipulate Noelle and Susie into capturing it and moving it into the living room and a scene requiring the SOUL to manipulate objects in Dess's closet while distracting Kris. Additionally, Chapter 4's data includes sprites and a functional test room for the toy train, while various sprites associated with the final sequence feature the phrase "homealone" in their filenames.

Within the data for Chapter 5 is an unused cutscene depicting Kris and Noelle riding the Ferris wheel at the festival after aborting the Weird Route in Chapter 4. Noelle recounts Kris' assurances that Chapter 2's events on the route were a dream before expressing how "amazing" it would've been if the power she demonstrated was actually real. When Noelle attempts to interrogate a reluctant Kris as to how they knew about the dream she had, she gets interrupted by Susie throwing dirt at the carriage window, which brings Noelle back to her senses. This cutscene is fully functional, but cannot be accessed without modding the game due to its associated requirements being impossible to meet in normal gameplay.

On July 11, 2026, both "Cutie Mew Mew Magic" and "Flower Man" from Chapter 5 became developer and composer Toby Fox's first two songs to reach the Billboard charts, used to catalog popular songs in the United States. Both songs appeared on the Hot Dance/Electronic Songs Chart, with the former debuting at No. 18 and the latter debuting at No. 13. While soundtrack albums for Deltarune's prior chapters previously appeared on the Billboard soundtrack charts, this marked the first time where individual songs from one of Toby Fox's games charted.

In a retrospective article detailing the making of Chapter 5, developer Toby Fox explained that the chapter's platforming mechanics were conceived shortly after Chapter 2's release in 2021, and were developed as a prototype before expanding in scope late into Chapter 4's development. According to Fox, a major issue with the mechanic's development was allowing it to mesh properly with the standard RPG gameplay of Deltarune, resulting in the formation of the ACT system and the addition of dialogue and RPG-style encounters in these sequences. The ACTs themselves originally encompassed a broad range of ideas before being whittled down to using Susie's Rude Buster and Ralsei's hanging ability. Fox also stated that the developers had to consider the possibility of Deltarune being many players' first experience with platforming gameplay, especially since Chapter 5, a relatively late point in the story, would be the first time it would appear in the game. Consequently, the level design was simplified to keep the "actual jumping challenges" from becoming too intense barring a few optional areas.

Within the files for Chapter 5 is an early draft of Noelle's portraits for Spoiler:the drowning sequence at the climax of the Weird Route. While most of these portraits feature slightly more pixel padding than their equivalents in the final game, the last four are substantially different in design. Additionally, the last two depict Noelle Spoiler:continuing to fade into the black background rather than cutting to a frame of her shouting euphorically. Additionally, the final versions of these portraits feature an unused final frame depicting Spoiler:a close-up of Noelle's eyes, likely intended for her final line of dialogue (which, in the released game, instead duplicates the shouting frame from the previous line).

Two tracks in Deltarune prominently interpolate "Merry CD", a track that developer Toby Fox contributed to the Omori soundtrack. The first is "d", which plays during the cutscene that precedes Spoiler:the Weird Route version of the first Berdly fight in Chapter 2. The second is "Gingerbread House", the theme that plays inside the Holiday family's mansion in Chapter 4. The choice of reference alludes to how both sequences prominently focus on Noelle, matching the holiday theming of the Omori song.

At the end of Spoiler:the Sword Route in Chapter 3, Spoiler: when Susie comes to greet Kris after the latter obtains the Shadow Mantle, the player is secretly given control of HERO_SWORD again. Consequently, it's possible to guide them back into the room and to Kris and Susie, which Kris notices. Attempting to attack Susie during this results in Kris pulling her away to protect her, whereupon she lets them taste her hair as compensation for her drinking their shampoo. Afterwards, Susie's dialogue proceeds as normal.