If the player attempts to buy a hotdog from Sans while their inventory is full, he will place a hotdog on the player's head. Sans will continue stacking hotdogs on the player's head until they have a stack of 29 hotdogs on their head.

The track "The Choice" almost exactly matches the track "Undertale" when sped up by 666%.

Temmie's weird speech patterns seem to be inspired by Mr. Saturn from the Mother series.

The markings seen on Papyrus' chestplate are the same markings seen on the Starman's chest from EarthBound.

A unused animation of the NPC species "Temmie" is seen walking or running. This may have gone unused due to temmie being a static character like most other NPCs.

An unused Spell icon was seen in a screenshot of the game, probably implying that spells were going to be used in the game.

There is an unused Sprite that is nearly identical to her death sprite. The file is named "spr_torielboss_suicide", possibly meaning that Toriel was originally going to commit suicide if they hurt the player too badly, or if Toriel lets the player out of the ruins.

Undertale has two secret endings that can be seen by doing a Spoiler:Pacifist Run after a Genocide Run. If the player chooses to stay with Toriel, Chara's face appears on Frisk and a lower pitched version of Flowey's laugh plays. If the player chooses to leave, the picture of Frisk and friends is replaced with a picture of Chara in Frisk's place and big Xs appear on other characters. This may be implying that Chara killed them after the game. The only way to get the true pacifist ending back is to either modify or replace the game's save files.

Prior to the release of the official Japanese version, Japanese audiences, who previously experienced the game through various fan translations, frequently debated about how Sans would refer to himself in dialogue. In the Japanese language, there are multiple first-person singular pronouns, each with their own distinct connotations. Among Japanese fans, the common consensus was that Sans would either refer to himself with 僕 ("boku") or 俺 ("ore"). While both pronouns are masculine, the former has an informal and "boyish" connotation, while the latter typically indicates brashness.

Once the Japanese localization came out, audiences were surprised to discover that Sans instead refers to himself with オイラ ("oira"), a relatively archaic pronoun which is commonly associated with hillbillies (though milder in connotation than おら, "ora"), only using "ore" during his sterner moments. The atypical choice of pronoun resulted in #オイラショック ("#OiraShock") becoming a trending topic on Twitter in Japan, with many fanartists in the country responding by making tongue-in-cheek drawings riffing on it.