Prior to January 20, 2016, there was an unused audio file within the game's named "abc_123_a.ogg". The audio contains several text-to-speech voices warning the player not to spoil the game online. This audio has since been replaced with a different clip laughing at the listener.

A crudely-made note exists in the game's sprite data, urging players not to post sprite sheets online until a year after the game's release, after everything normal has "been seen in context first." The note makes mention of VGFacts partner website "The Spriters Resource" specifically. This note was replaced with a different image following the January 20, 2016 update and removed entirely in later versions.

According to Tobyfox, most of the inspiration for Undertale came from Shin Megami Tensei and the Mario and Luigi RPG games.

"The inspiration for the talking system came from Shin Megami Tensei. I feel that it's important to make every monster feel like an individual. If you think about it basically all monsters in RPGs like Final Fantasy are the same, save for the graphics. They attack you, you heal, you attack them, they die. There's no meaning to that. The inspiration for the defensive segments came from the Mario & Luigi RPG games and bullet hell shooters like Touhou. Yes, the system will get more interesting. I don't want anyone not to be able to play the game because they aren't some bullet-dodging fanatic."

Papyrus and Sans are named after the fonts they speak in (Papyrus and Comic Sans, respectively), an idea inspired by Helvetica, a webcomic about a skeleton named after a font.

If the player attempts to name the protagonist "Papyru", the game responds with "I'LL ALLOW IT!!!" in Papyrus' text, Spoiler:making Papyrus one of the few boss characters whose name can be used for the main character, although the character limit prevents the player from entering the last letter.

The other is Mettaton, although obviously his name is too long for the input screen, inputting the name "Metta" or "Mett" will not only be allowed by the game, but the game (representing Mettaton) will ask if the player is promoting his brand.

Sans' Theme, MeGaLoVania, is a song made by the Developer for both a Halloween romhack of EarthBound, and also appears on the album "Homestuck Vol. 6: Heir Transparent". It was inspired by the boss theme from Live-A-Live, "Megalomania".

Toriel was created as a parody of tutorial characters, with developer Toby Fox citing Fi from The Legend of Zelda: Skyward Sword as a notable influence. Her motherly personality was chosen both as an extension of her hand-holding and due to the general lack of genuinely motherly characters in RPGs.

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.