Not exactly a mondegreen, but there was the story going around about the Beatles' song, Baby You're a Rich Man... It was cobbled together from a couple of disparate song fragments that John and Paul were each working on separately, the Lennon bit supposedly written for Brian Epstein (to tell him to relax now; he was rich)

The lyrics go:
Baby, you're a rich man
Baby, you're a rich man
Baby, you're a rich man too...

But some claim that John Lennon sings on one of the choruses: Baby, you're a rich fag jew...

I confess I can't hear where it's supposed to be in numerous close listenings to the song. But then again, I can't hear the infamous "I buried Paul" line either. Though I have heard other bits in other songs where Lennon obviously flubbed his lines.