We've got a bloke at my folk club who does this on purpose, usually to well know songs, so now Bonnie Annie Laurie has "Little Furry feet" and the chorus to "Black velvet Band " goes "with her hairy tongue over her shoulder." He's wrangled the line "With sheep in your wellies" into Mull of Kintyre as well.

Accidental ones, my father genuinely though Whitesnakes "Ain't no love in the heart of the city" was "Anal love in the heart of the city." Ah well.