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

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.

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

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

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

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.

Napstablook's name may be a play on "Napster bloke," in reference to the file-sharing software Napster, which was frequently used to share music files. The computer in Napstablook's house also displays "a music-sharing forum".

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.

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

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

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.

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.

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.

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.

The game contains an ending error message where Sans will say;
"i'll be honest.i have no idea what happened for you to get here. this is actually some sort of error- handling message. so, if you're getting this ending...tell whoever made the game, okay? they'll fix it, or if it's a novel situation... they might even add another ending to the game. chances are, though... you're just a dirty hacker, aren't you? yeah, get outta here."

This works as an error handler when none of the available endings are triggered, which shouldn't happen during regular gameplay.

According to developer Toby Fox, the song "Redline Day" helped inspire him to make changes to the Undertale OST.

There are unused graphics of a robot NPC in idle and talking animation stored in the game's memory.

During then credits after the Spoiler:pacifist run, if you dodge everything, yellow text will appear saying "Incredible work! Somewhere, felt a door opening...!". This unlocks a door in Snowdin Forest.

The song "Pathitic House" sped up sounds similiar to the song "Spider Dance".

The song "Dating Start!" sounds almost exactly like the song "Turnabout Sisters" from Pheonix Wright: Ace Attorney