Colossal collaboration failures

I was just thinking about some of the weirdest duets I've heard in my life, and wondering why some of them work shockingly well and others are even worse than what you could've possibly imagined. A notorious trainwreck collaboration is obviously Lou Reed with Metallica.

So I thought I'd try to start a discussion about cursed duets. What are your favorite WTF pairings?

Just to add a bit of challenge to this discussion, let's just automatically disqualify "rap and metal" duets up front. Those haven't been "unexpected" since Run DMC recorded "Walk This Way".