4x21 - The Grim Adventures of Billy and Mandy

When Billy buys a new cereal, a phoenix emerges from the box and attaches to his head, annoying everybody. Since the phoenix cannot be removed because it revives every time it is destroyed, Billy, Mandy and Grim seek help by going to the factory where the cereal is produced, which is run by Eris, the goddess of chaos. Mandy tricks the phoenix into leaving Billy and attacking Eris instead. It is revealed that all the events were really an episode-within-an-episode.