From the Nobody Ever Tells Me Anything Dept.: Did you know that three of the songs on Kiss' dreadful 1981 album Music From "The Elder" were co-written by Lou Reed? According to Kiss legend, Reed contributed the line "A world without heroes is like a world without sun" to the song "World Without Heroes," which was the album's first (and only) single. But the album credits claim that Reed co-wrote "Mister Blackwell" and "Dark Light" as well.
That's not the most bizarre songwriting collaboration out there, though. Not until someone shoots down the story that Bob Dylan co-wrote "Steel Bars" with Michael Bolton. I can't verify the provenance of this, but this quote supposedly comes straight from Bolton:
"We're planning on writing some more songs together. He's kind of hungry to get back out there and wants to work with a few contemporary hit song writers. Someone who works with Dylan called me up and said 'Bob Dylan would like to write with you.' I was awed. I told him, 'I don't even know how I could write a lyric when working with you ... I'm too intimidated.' But then we started messing around with some chords and wrote 'Steel Bars,' a song about obsession. It took us two sessions to write, and when I left, I was told, 'Bob likes you and he wants you to come back.'"