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

If you attack the dummy inside of the ruins, but attack and miss it nine times in a row, a text will pop up saying "Dummy tires of your aimless shenanigans." This will automatically end the battle, earning you 0 gold and 0 XP.

The song "Death By Glamour" is a remix of the songs "Core", "Metal Crusher", and "It's Showtime".

The game contains an unused battle with "RG 03" and "RG 04", which appears to be mostly complete with the exception of using placeholder graphics.

In the game's demo as well as full version prior to patch v1.001, an unused song by the name of "grandpasemi.ogg" could be found in the game's files. This was likely intended for a cut character that could have been related to Sans, Papyrus, and possibly even W.D. Gaster, due to the name Semi being based on Semi Serif fonts. It is a short, slowed-down, and re-instrumentated version of the song Metal Crusher, which is used as Mettaton's first battle theme.

This same song is now used as of patch v1.001 when you deny feeding Temmie any Tem Flakes. It plays while Temmie makes a unique facial expression, with a text graphic appearing beside her that says "can't blame a BARK for tryin'...", immediately followed by Temmie trying to attack you with her extendable arm.

Following the January 20, 2016 update, the note within the game's files urging players to wait at least a year before posting sprites online was replaced by a new note reading "you know what i HATE? that's bepis THE TASTE the smell the texture hey.... your drooling......".

This is a reference to a man in Moonside from EarthBound who hates the food the player likes. Creator Toby Fox is a fan of EarthBound, and Undertale was heavily influenced by it. The word "bepis" itself is a slang term for "penis" that originated as a mispronunciation of the word and later evolved into an online meme.

The opera scene with Mettaton is likely a parody of "Aria di Mezzo Carattere" from Final Fantasy VI.

By speeding up the song "The Ruins", it is almost identical to the songs "NGAHHH!!" and "Spear of Justice".

The track "Waterfall" almost exactly matches the melody of "Spear of Justice", but at about a third of the speed.

Spoiler:After beating the game by only killing Toriel, Asgore, Papyrus, Mettaton, and Undyne, Sans will call the player after the Omega Flowey fight to tell them that the annoying dog became president.

While meeting Sans for the first time, Spoiler:Sans will ask the protagonist to turn around and shake his hand, which has a whoopie cushion hidden in it. However, if you immediately exit the game without saving and then meet him again, he will ask you if you've heard his joke before, since the main protagonist turns around before being asked.

The character W.D. Gaster shares some visual similarities with the character Uboa from Yume Nikki, another RPG-based title. Their appearance is also similar to Dr. Andonuts from Undertale creator Toby Fox's Halloween Hack for EarthBound.

The song "But Nobody Came" sped up 1600% sounds similar to another Undertale song, "Your Best Friend".

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

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.

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.

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

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

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."

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.