Snail

The Snail is a character that the creators of Adventure Time hide in every episode (excluding "Trouble In Lumpy Space" and "Blood Under the Skin"). It is seen making cameos, usually waving at the screen. Its cameos are a running gag.

The Lich
While the Snail was just a running gag at first, he eventually released The Lich, the most powerful evil in the Land of Ooo. In the episode "Mortal Folly", he comes out of Finn's bag to wave to viewers, then falls under The Lich's mind-controlling spell and releases him from his amber prison. He appears again at the end of "Mortal Recoil" still being controlled by the Lich.

As of season three, the Snail continues to appear in each episode, and (when examined closely) appears to still have an evil expression on his face.

Season One
The tables below list the Snail's known whereabouts.

Trivia

 * In Righteous Quest 2, the Snail appears a couple times in the game.
 * The Snail starred in its own FusionFall game: "Snail Quest".
 * It's running gag of appearing at a certain moment, usually hidden, could be a reference to "Where's Waldo?". It's also similar to Pandaman of One Piece series.