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.
After 6 seasons, Warner Bros green lit an Adventure Time film adaptation, making it the first Cartoon Network intellectual property to make it from the small to big screen since The Powerpuff Girls Movie (2002).
The trivia items below may give away important plot points.
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 the episode "Dark Purple" Jake predicts the coming events. Jake says if they go to on an adventure, it would result in them finding a conspiracy, fighting a bad guy, and ruining their favorite drink. It then cuts to Susan Strong, who goes on an adventure that results in exactly that. At the end, Jake says he jinxed it.