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 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 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 Nintendo Switch version of the game, the Dog Shrine from the PlayStation 4 and PlayStation Vita versions appears abandoned and in shambles. The only new addition is a red and blue door referencing the Switch's Red and Blue variant Joy Cons which are used to open it. The door can be opened Spoiler:following the Mettaton EX boss fight and allows players to access a boss fight exclusive to the Switch version, Mad Mew Mew. Spoiler:This character is implied to be the spirit of Mad Dummy after it possessed a "Mew Mew Kissy Cutie" doll belonging to Alphys.

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.

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

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

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

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.

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.

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

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

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

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.

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

Naming the fallen child a certain name 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 developer Toby Fox, the song "Redline Day" helped inspire him to make changes to the Undertale OST.

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.