http://en.wikipedia.org/wiki/Mondegreen
Mondegreens are always good for a larf, but nothing beats the one that coined the phrase:
When I was a child, my mother used to read aloud to me from Percy’s Reliques, and one of my favorite poems began, as I remember:
Ye Highlands and ye Lowlands,
Oh, where hae ye been?
They hae slain the Earl Amurray,
And Lady Mondegreen.
The actual fourth line is “And laid him on the green.” As Wright explained the need for a new term, “The point about what I shall hereafter call mondegreens, since no one else has thought up a word for them, is that they are better than the original.”