Not really a Mondegreen, but for a really poor attempt to extricate the singer from a forgotten-lyrics crisis, I nominate Pete, then lead singer of Rigid Fish, a fairly incompetent rock band I formed with some university friends. Amidst our standard fare of Groundhogs and Status Quo covers we performed (for reasons that remain puzzling) 'King Henry' as recorded by Steeleye Span. Somewhere in my archive there is exists a recording proving that Pete, in place of the line: 'In there came a grizzly ghost, stamping on the floor', actually sang: 'In there came a thingy thing, wanking on the stairs'.

Ah, the power of those old Child Ballads....