The skeleton brothers, Papyrus and Sans, are named after the two typefaces, Papyrus and Comic Sans Serif, which is also the typeface used for their dialogue in the game.
After meeting Flowey at the start of the game, if you backtrack to certain points you will catch brief glimpses of Flowey, before he burrows back underground.
Toby and Temmie, the two primary designers of the cast of NPCs and Monster encounters in Undertale both have some form of alter ego in the game. Toby takes the form of the Annoying Dog, a Pomeranian-based dog that usually appears in moments that playfully torment the player or other characters, and Temmie is represented by a cat-dog hybrid named Temmie, that speaks with an odd speech pattern.
Sans' voice is actually just a small soundbite played over and over again of Bill Fagerbakke, the voice of Patrick Star in Spongebob Squarepants.
Matthew Patrick, creator of the popular YouTube series, Game Theory, gave this game as a gift to Pope Francis.