To mushroom is to, when singing along to a
song, either mumble or sing the
wrong words, revealing that you don't really know how the song goes. When someone does this, everyone around them yells "MUSHROOM!" at them, as a way to embarrass them.
Matt: If you wanna be my lover, you have got to give....doo doo doo...but that's the way it is!!
Nick: Oh
Matt, you totally just Mushroomed!
Everyone in car:
MUSH-ROOM! MUSH-ROOM!