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

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.

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.

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

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."

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.

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".

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.

There is a mock up drawing of Omega Flowey (Photoshop flowey) showing that he was originally going to carry the souls rather than them being in his petals.

Attempting to name the fallen child "Gaster" will inexplicably reset the game. Spoiler:This ties into a subplot revolving around the unseen character Dr. W.D. Gaster.

A secret character who is hinted by people like the River Person is W.D. Gaster. According to clues scattered throughout the game, he was the royal scientist before Alphys and was responsible for creating the core, but he fell into the creation (which is implied to be the core, but could be the strange machine in San's secret workshop) and his life was cut short, disappearing from existence, but his consciousness is still present. Other than minor hints, trying to find W.D. Gaster and direct references to him can't be found playing the game normally, but can only be found if you edit the game's files and fun values. Also, if you go to your save file by editing the game files as well, you could also get a message from W.D. Gaster himself in the WingDings font (which is who he is named after), which he talks about his research, and then the game will crash.

Spoiler:There are two alternate ways that Muffet will spare the player. If the player fights Muffet through pacifist mode, and eats a spider doughnut in front of her. Or if the player buys either a spider doughnut or spider cider for 9999g.

Spoiler:Asriel Dreemurr's name is an anagram of "serial murderer".

The games files contain graphics for an unused "Doge" enemy.

Following patch 1.001, the unused audio file "abc_123_a.ogg", originally warning the player not to spoil the game online, was replaced with several text-to-speech voices laughing at the listener.