Undertale/Trivia

Contest Winner Cameo: Glyde, So Sorry and Muffet were designed by Kickstarter backers, and so were a few NPCs.
Defictionalization: Because of the vast amount of healing items in the game, nearly every food in the game has at least one fan recipe. Spider cider donuts (sans actual spiders), Toriel's cinnamon-butterscotch pie, Papyrus-inspired spaghetti, spicy Grillby-inspired fries, Sea Tea, etc. At least one Tumblr is trying to actually compile a fan-made Undertale cookbook!
Executive Meddling: Sony mandated, among other things, that trophies be in the PS4 and PS Vita versions of the game. Toby Fox hated the idea of Video Game Achievements, feeling they undermine the whole point of the game. Fortunately, they didn't tell him what had to be in those trophies, so he basically wrote what can only be described as Writer Revolt, and pretty much all the trophies are things you can get no matter what path you take.
Fan Community Nickname: Fans often call themselves "Undertale Trash". Obviously an Appropriated Appellation, starting out as an insult from those in Hype Backlash towards the game, but it has ironically been adopted by the fans themselves.
Fan Nickname: "Goatmom" and "Goatdad", for Toriel and Asgore respectively, "Goatbro" for Asriel.
He Also Did: Before making Undertale, Toby Fox was known for composing much of the music used in Homestuck. Everdraed, too, is mostly recognized for making extremely complex photo-manipulated gifs (often with embedded sound) on SomethingAwful, YouTube and Twitter, so his work on the Neutral final boss comes as little surprise.
Sleeper Hit: Not even Toby himself expected the critical success Undertale would get when he released it.
Working Title: During the Kickstarter period, the game's title was capitalized differently, being seen as both "UnderTale" and "UNDERTALE". Eventually, it morphed into "Undertale" and stayed that way.
The Wiki Rule: Undertale Wiki (Wikia) and Undertale Wiki (Gamepedia)