On the David Bowie album Heathen, I messed up the lyrics for two of my favourite songs:
On the song Cactus, it's actually "Sitting here (and this word I'm still not sure about, although I think it's "wishing") on a cement floor..."
I won't tell you what I originally thought "cement" was.
On the next track, cos I forget the name, I heard "Twinkle, twinkle, Uncle flied" and it turned out he was saying "Floyd." Now how on earth did I mess that up?