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.

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".

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.

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.

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."

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.

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.

Occasionally the River Person's boat will have a dog figurehead on it, and the boat itself will then run across the water rather than float.

Attempting to give the main character a name already in use will prompt a unique response from that character. While certain enemy names are acceptable, the names of story-important characters are forbidden.

Spoiler:The recurring character, named Annoying Dog, is a representation of the game's creator, Toby Fox.

When Undertale was announced as a kickstarter project it was originally aiming for $5,000 to create the game. The campaign succeeded by going over that with $51,124 with the help of 2,398 backers.

The song known as "Dogsong" was made by using the soundfont from Mario Paint's music mode, although some notes in the song are too low to recreate it in the normal music mode.

The spider boss Muffet was designed by Michelle Czajkowski, creator of the webcomic Ava's Demon.

Originally it was planned for Spoiler:Mettaton to save images of your essays to your hard drive, the feature was removed from the final version because it was too buggy.

In the fight with Spoiler:Sans, if you try to spare him after he spares you, he offers you a hug. After the dialogue boxes that arise afterwards, you get attacked by undodgeable white bars and get a Game Over screen with Sans saying "geeettttttt dunked on!!!".

It's possible to die in the fight against Toriel. Accomplishing this causes her to show a shocked expression on her face for a split second before going to the Game Over screen.

According to artist Tuyoki/Temmie Chang's Tumblr page, the appearance of the Temmie race was based on a simple doodle done by another artist who didn't know what she looked like. Tuyoki made the doodle into her mascot and used the Temmie race as her representation within Undertale.

There is a hidden ending that can only be accessed by skipping directly to the credits. In it, Sans says that it is most likely an error handler, but that the player probably got there by cheating. He then calls the player a "dirty hacker."

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.

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.