In an interview conducted to promote the 2025 remaster of Off, Toby Fox cited the original 2008 PC release as an influence on Sans and Papyrus in Undertale. Sans was inspired by both Zacharie and the Judge, specifically noting the former's comic relief status, constant smiling, and ambiguous motives. Meanwhile, Papyrus' design was originally based on Dedan, as indicated by early concept art which Fox posted on Twitter. Additionally, Fox cited Off's jazz-driven soundtrack as an influence on the song "Ghost Fight" (and, by extension, "Dummy!" and "Mad Mew Mew").

One of Toriel's sprites, a facial expression of her with an uneven frown and an eyebrow raised slightly higher than the other, is internally called "dreamworks", referencing the "DreamWorks Face", a nickname for a facial expression comprised of a confident, uneven smirk and one eyebrow being raised while the other is down. It is named as such for the film studio DreamWorks Animation, which frequently uses this facial expression in promotional artwork for its movies.

The Spoiler:Amalgamate enemies appear to take after an identically-named series of foes from The Halloween Hack, a 2008 ROM hack of EarthBound that Toby Fox made in high school; like the ones in Undertale, Spoiler:the Amalgamates in The Halloween Hack are comprised of multiple earlier enemies fused together.

Furthering the connection to the EarthBound fanbase, Spoiler:"Amalgam", the battle theme for most of Undertale's Amalgamates, is composed mostly of samples from the Cave of the Past and final boss themes in EarthBound, including the processed sample of the Beach Boys' 1970 song "Deirdre".

The tracks "Bonetrousle" and "Heartache" were originally composed for an early idea for Deltarune (whose gestation predated that of Undertale) before being repurposed here. "Bonetrousle" was initially the main battle theme for Deltarune before being replaced with "Rude Buster", while "Heartache" was originally titled "Joker Battle", implying that it was initially the battle theme for Spoiler:Jevil before being replaced with "THE WORLD REVOLVING".

The Japanese version of the game redubs Flowey's "That's a wonderful idea!" voice clip to a Japanese equivalent. In a DM sent on Twitter to VTuber Houshou Marine, Undertale creator Toby Fox confirmed that he voiced the Japanese line himself.

In the Xbox One and Xbox Series X/S versions of the game, the Dog Shrine is bulldozed and a Dog Casino is built in its place, containing a similar slot machine side quest that can unlock decorative items and 11 Xbox achievements. One of those unlockable items is a poker table featuring Sans, who can comment on your prizes found in the room. Spoiler:During a Pacifist run following the final boss fight, the player can return to the Dog Casino to find Mad Mew Mew in Sans' place, who will also comment on your prizes.

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.

In the PlayStation 4 and PlayStation Vita versions of the game, there is a secret area Spoiler:hidden underneath the sink in Sans & Papyrus' house called the Dog Shrine. Playing through the entire donation side-quest contained within, in which the player gradually gives enough gold to a Dog housed in a Japanese-style shrine, awards the player a total of 15 PSN trophies and numerous prizes the player can interact with in the room.

The song from the OST, "She's Playing Piano" by Toby Fox seems to be a rendition of the popular webcomic series Homestuck's "Doctor." Toby Fox would eventually make a scrapped remix of the song titled, "Patient," which was originally intended for Homestuck but was remixed into Undertale's "Another Medium."

The song "Hopes and Dreams" is comprised of various songs heard throught the game; some examples include "Snowdin Town" and "Your Best Friend".

The unused music track "mus_smile" is actually Muffet's laugh, but slowed down with its pitch lowered.

If you listen closely, the music played after getting a game over can be heard halfway through the song "ASGORE". Spoiler:This is fitting, as Asgore is the only main character in the true pacifist route that the player must kill.

In the neutral route, Spoiler:if Flowey closes your game after killing Asgore, when you load up your game again the save file's name will be changed to Floweytale.

When an enemy without a death sprite dies, it will use Aaron's death sprite as place-holder for any enemy without one.

Naming the fallen child a certain name used by another character will give you unique messages after you chose it. Some names will also do certain things. Here is a list of names that give you unique messages and do certain things:

• Flowey - Message will say "I already CHOSE that name" and prevents you from using that name.
• METTA/METT/MTT - Message will say "OOOH!!! ARE YOU PROMOTING MY BRAND?"
• Alphys - Message will say "D-Don't do that" and prevent you from using that name.
• Alphy - Message will say "....OK????"
• Toriel - Message will say "I think you should think of your own name, my child" and prevents you from using that name.
• Sans - Message will say "nope." and prevents you from using that name.
• Papyru - Message will say "I'LL ALLOW IT!!!"
• Shyren - Message will say "...?".
• Asriel - Message will say "..." and prevents you from using that name.
• Napsta or Blooky - Message will say "............ (They're powerless to stop you.)"
• Frisk - Will initiate hard mode.
• Temmie - Message will say "hOI!"
• Aaron - Message will say "Is this name correct? ;)"
• Woshua - Message will say "clean name".
• Bratty - Message will say "Like, OK I guess".
• Catty - Message will say "Bratty! Bratty! That's MY name."
• Asgore - Message will say "You cannot" and prevents you from using that name.
• Gerson - Message will say "Wah ha ha! Why not?".
• Chara - Message will say "The true name". Removing the name from a save file will make the name be given by default.
• Gaster - Causes the game to go back to the intro, making the game not let you use that name without hacking.
• Bpants - Message will say "You are really scraping the bottom of the barrel".
• Jerry - Message will say "Jerry".
• AAAAAA - Message will say "Not very creative...?".
• Murder or Mercy - Message will say "That's a little on-the-nose isn't it...?".
• DrakGigiGugu - The item "Steak in the Shape of Mettaton's Face" will show as FSteak in the inventory instead of showing as Face Steak.

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.

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.

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!!!".

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.

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