As a running gag, in every episode (Except in the episode Food Chain ), a small snail can be found waving at the viewers. The snail is usually only on screen for one scene per episode. The preschool show "Blue's Clues" also features snails hidden throughout each episode; perhaps the inspiration.
The first series produced by Frederator Studios to not air on Nickelodeon. Pendleton Ward originally pitched it to the network twice after doing the pilot for them which aired on Random Cartoons, but was eventually rejected both times before Ward pitched the show to Cartoon Network and got greenlighted.
Some of the characters of the show are survivors of the Great Mushroom War. Marceline was a child sometime after the war, and the Ice King, who was then known as Simon Petrikov and was slowly changing into his insane personality from wearing a powerful crown he got from a Scandinavian fisherman named Gunther.
In one episode, Jake predicts the coming events. In the episode dark purple, Jake says if they go to on an adventure, it would result in them finding a conspiracy, fighting a bad guy, and ruining there favorite drink. It then cuts to Susan strong, who go's on an adventure that results in exactly that. At the end of the episode, Jake says he jinxed it.