A mondegreen is a mishearing or misinterpretation of a phrase in a way that gives it a new meaning. Mondegreens are most often created by a person listening to a poem or a song; the listener, being unable to clearly hear a lyric, substitutes words that sound similar and make some kind of sense.
This song just came on randomly on YouTube:
It sounds like Emma Bunton at 36s in is singing "What's driving you is I'm pished and I'm betting".
What other misheard lyrics spring to mind?