According to developer Toby Fox, the song "Redline Day" helped inspire him to make changes to the Undertale OST.
Contributed by Countnecula
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.
Contributed by NikesLP
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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.
Contributed by NikesLP
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.
Contributed by game4brains
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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.
Contributed by Ophl
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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.
Contributed by CLXcool
Spoiler:Asriel Dreemurr's name is an anagram of "serial murderer".
Contributed by MeleeWaluigi
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The games files contain graphics for an unused "Doge" enemy.
Contributed by MeleeWaluigi
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.
Contributed by Kitsune Hawk
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".
Contributed by Kitsune Hawk
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.
Contributed by MeleeWaluigi
The track "The Choice" almost exactly matches the track "Undertale" when sped up by 666%.
Contributed by Countnecula
Temmie's weird speech patterns seem to be inspired by Mr. Saturn from the Mother series.
Contributed by MeleeWaluigi
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The markings seen on Papyrus' chestplate are the same markings seen on the Starman's chest from Earthbound.
Contributed by Boyobmas
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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.
Contributed by NikesLP
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An unused Spell icon was seen in a screenshot of the game, probably implying that spells were going to be used in the game.
Contributed by NikesLP
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NSFW - This trivia is considered "Not Safe for Work" - Click to Reveal
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.
Contributed by NikesLP
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.
Contributed by Cavery210
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."
Contributed by PoisonTardis
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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.
Contributed by Kitsune Hawk
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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.
Contributed by PowerRing
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!!!".
Contributed by JomasterII
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.
Contributed by ThisGuyInTheSuit
The spider boss Muffet was designed by Michelle Czajkowski, creator of the webcomic Ava's Demon.
Contributed by gamemaster1991
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.
Contributed by G-Haven
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.
Contributed by CLXcool
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Spoiler:The recurring character, named Annoying Dog, is a representation of the game's creator, Toby Fox.
Contributed by Smirkytrick
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.
Contributed by Kitsune Hawk
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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.
Contributed by PowerRing
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.
Contributed by Kitsune Hawk
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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.
Contributed by Kitsune Hawk
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."
Contributed by CLXcool
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.
Contributed by Kitsune Hawk
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.
Contributed by Kitsune Hawk
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".
Contributed by NikesLP
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.
Contributed by Kitsune Hawk