Don't know why but that made me laugh a lot.
The earliest one I can remember was sing "Roll Alley 4" instead of "You're all I live for" to Alvin Stardust's 'You, You, You'. I genuinely thought he was singing about how much he loved ten pin bowling.
Listening to The Undertones on 6 Music has just reminded me of one of my favourite childhood lyric malfunctions; I used to sing "he thinks that I'm a Chaffinch, 'cos I hate University Challenge" to My Perfect Cousin.
Got any goodies?