The character of Mabel is based on Alex Hirsch's twin sister, Ariel. Alex Hirsch used his own grandfather for the inspiration for Grunkle Stan. Soos is based on an old college buddy of Alex Hirsch's named Jesús Chambrot.
Get 'em! Get 'em!
Throughout the series, the Dinosaur Skull's (Probably a Tyrannosaurus skull) Fenestra (Hole in front of the eye hole) keeps changing shape along with its location on the skull.
All the episodes (except weirdmageddon 3:take back the falls) contain an extra scene during the credits that usually continues something from what happened earlier depending on what episode.
Louis C.K. originally made a cameo in two of the Weirdmageddon episodes as The Horrifying Sweaty One-Armed Monstrosity. But after C.K. admitted to sexual misdemeanors, Alex Hirsch re-dubbed the character. All future airings will be of these versions of the episodes.