My favorites is not my own mondegren, but one made by professional musicians.
When Joan Baez sang “The Night They Drove Old Dixie Down,” she changed the line “Till Stoneman’s cavalry came and tore up the tracks again” to “Till so much cavalry came.”
Speaking of which, isn’t “Innagadda-da-vida” (or however that’s spelled) a mondegreen itself? I seem to recall seeing an interview on MTV saying that the line was actually “In the Garden of Eden” but one of them misheard it when another sang it for the first time, and Innagadda-da-vida was born.
Queen, Bo Rap
I thought for years Freddy was singing “gotta leave you all behind and fade away.” Now I know that he’s actually singin “face the truth” but I still prefer mine.
I’ve never been certain whether the Killer Queen is faithful or graceful as a pussycat. Sometimes I hear one, sometimes the other.
According to Mr. Punky, the original lyrics were “in the Garden of Eden,” but this wasn’t acceptable. I’m not sure if it was the record label, the radio stations or the FCC, but someone said they couldn’t use those words. The band changed the lyric to “Inna-gadda-da-vida,” and that passed muster.
I’ve since been assured that this is true by several other people, but it sounds a little fishy to me. Why would the Garden of Eden be unacceptable? Who censored lyrics? By the time this was recorded, wasn’t everybody falling all over themselves to be “daring?” But it seems to make some people happy to think they are listening to something forbidden, and my questions have been greeted with hostility.
And yet you never hear playful? 
That’s not how I heard the story told. I heard it was supposed to be In The Garden Of Eden, but he was so wasted when he was writing it that it came out In-A-Gadda-Da-Vida and the let it stick.
Maybe that wasn’t a mistake. Didn’t she change some other lyrics in that song too?
So says Allmusic.com:
:smack: d’oh