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.

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.

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.

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 and removed entirely in later versions.

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."

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.

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.

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".

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.