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 early 2026, complaints arose from Latin American fans regarding the lack of official Spanish and Portuguese localizations of Undertale and Deltarune. This stemmed from earlier complaints over merchandise producer Fangamer's high shipping costs to Latin American countries, and a lack of South American tour dates for the "Undertale: The Determination Symphony" concert tour, both due to financial and logistical issues on part of Fangamer and the concert organizers.

In response, creator Toby Fox released a statement explaining that the reason why the games' only official translations were in English and Japanese was because he wanted to provide direct oversight to localizers in languages he was already fluent in. Since the games' scripts rely heavily on wordplay, he wanted the translations to adhere as closely as possible to his intended vision to reduce the risk of translation errors, adding that he was "not against official translations for other languages if we could make it work the same way..." Regarding Deltarune specifically, Fox expressed concerns that producing more translations would prolong the game's already drawn-out development, and endorsed unofficial fan translations as a way to allow fans to "interpret the game without the burden of being "officially" perfect". This statement drew mixed responses on social media, with some defending his perfectionist creative vision and efforts to manage the scope of development, while others felt that his standards were unnecessarily high and that the financial success of the games justified the demand for more translations.

In a retrospective article detailing the making of Chapter 5, developer Toby Fox revealed that Seth was originally designed with lavender skin and white hair. The decision to revise this idea came about after his friends pointed out their resemblance to the character Amethyst from the 2013 American animated series Steven Universe. Concept art featured in the article showcases an early alternate design that features a uniformly white appearance barring a purple flower collar and a purple crescent moon decal on Seth's robe. A second design depicted in the same art features the same purple palette depicted in the final game, but retains the collar. While Seth's final design drops the decal and replaces their collar with a tie, the sprite based on the white design's pose features the tie curving into a crescent shape, alluding to this early idea.

During the final battle of Chapter 3, Spoiler:the Roaring Knight is able to render Susie and Ralsei's HP to -999 when knocking them out, with the results represented by red text reading "SWOON". This appears to reference the Black Sword weapon in Final Fantasy IV, which has a chance of knocking out enemies in one hit. In the English localization of that game, the text "Swoon!" shows up when this occurs. The Knight further references this phenomenon with its own sword, the Black Knife (which also serves as its battle theme's namesake).

From Chapter 2 onwards, the game's title screen includes a small subtitle below the main logo listing the current chapter. In addition to including graphics for Chapters 2 through 7 (lining up with the visible options on the chapter select screen), the game's data also includes Spoiler:graphics for Chapters 0, 1, 8, 9, and 10. While the Chapter 1 graphic is presumably for organizational purposes, the role of the remaining unused graphics is more ambiguous. As the game states that "THE HALFWAY MARK HAS BEEN ATTAINED" after completing Chapter 4, it is unclear whether the references to Chapters 0, 8, 9, and 10 were reflective of an earlier, longer outline for the game's story or if they were included as red herrings.

Within the game's data is an unused "Talk" button for the Dark World's overworld menu, intended to allow the player to hear observations from Kris' party about the current area, similarly to the tattle feature in the Paper Mario series. The game's code still contains a script, scr_talkroom, which points to this scrapped dialogue, though the only remaining text is a single placeholder line: "Your voice echoes aimlessly." The Talk button also still has an icon and associated code placing it in the center of the menu, although the released game is programmed to omit it from display.

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.

Chapter 5 features many references and similarities to the Touhou Project series, of which developer Toby Fox and illustrator Temmie Chang are fans, through outfits, items, character designs, in-game text, and music. These include, but are not limited to, the following:

• The Red Ribbon and Netskie Hat headgears are references to Reimu Hakurei and Ran Yakumo.
• Aqua's character design, which Fox admitted was directly inspired by the Touhou series, bears similarities to characters such as Cirno, Sakuya Izayoi, and Koishi Komeiji.
• Flowery's line "Star sign: the Flower", presented like a dating simulator profile, is formatted similarly to Spell Cards' names, while bullet patterns throughout the chapter resemble Spell Card patterns.
• In the Ideal Diner, a Floradinn repeatedly orders eggs with increasingly hardening yolks, referencing difficulty options and stages in the Touhou games.
• The sound effect for collecting Flowery Dollar coins is a whispered voice resembling the graze sound effect from the licensed fangame Touhou Luna Nights.
• The Phanta item's description calls it a "grape-flavored phantasmagoria of a soda's dream", referencing Touhou Yumejikuu: Phantasmagoria of Dim. Dream. Its drink dispenser description, "Purple soda in hidden flavor", and ability to grant 16 TP allude to the 16th mainline game in the Touhou Project series, Touhou Tenkuushou: Hidden Star in Four Seasons.
• In Yellow's first punishment room, the party can encounter Shinobeetles trapped in quicksand asking "(Won't somebody please help me?)". This references a 2014 tweet by Fox about a presentation slide in a class lecture featuring a photoshop edit of Sakuya sinking in quicksand.
• The chapter's soundtrack quotes several Touhou themes, including "Theme of Eastern Story", "Lovely Mound of Cherry Blossoms ~ Japanese Flower", and "The Doll Maker of Bucharest". "Petal Dance," the battle theme for most of the colored flowers, prominently features Romantic Tp, a trumpet soundfont commonly used in the Touhou games and nicknamed the "ZUNpet" by fans. The "Shop 3" theme mimics the sound chip featured in the PC-9800 Touhou games.
• There are two references exclusive to the Japanese version of the game. Bullet patterns are referred to as "Danmaku", a term used for them throughout the Touhou series, while Flowery's "smile again" voice clip references a similar line from the Touhou fan video "My Name is Hong Meiling".

In a retrospective post detailing the making of Chapters 3 & 4, director Toby Fox revealed that the layout of Chapter 3 underwent multiple revisions over the course of development. Fox initially started with eight different segments (four boards and four minigames) which then expanded to 17 with the inclusion of additional rooms and interludes. The final game would trim it down to 12, with the third envisioned board (which would've featured a "city" theme) being removed entirely due to it not working with the game's overall style. Vestigial remnants of this axed board remain in Spoiler:the Sword Route, where a stripped down version of it appears near the end of the Original Game. A cowboy-themed minigame also ended up removed, with the final game distilling it heavily to a shooting gallery. According to Fox, this fluctuating approach was due to him wanting to keep Chapter 3's pacing from getting bogged down while still retaining the original idea of it being a homage to the Mario Party series and the Strange Fortress stage in Gunstar Heroes.

The various changes made to Chapter 3's layout are reflected in unused content left behind in the game's data. Among other things, the game's code still contains assets from the full version of Board 3, early versions of other boards and rooms, and corresponding unused dialogue, music, and graphics. The latter categories include both early versions of material found in the final game and assets that were removed altogether.