This title in a NYtimes article well describes Mondegreens. For the uninitiated see:

http://cs.anu.edu.au/~Carol.Edmondson/cc/mondegreens/MoreMondegreens.shtml

and

http://www.sfgate.com/columnists/carroll/mondegreens.shtml

My own was from my first years in primary (elementary) school when we sang the (UK) national anthem:

"...long to rain over us
God save the Queen."

and had some difficulty picturing her Majesty gently falling from the sky!

Perhaps you might have your own Mondegreen(s) to share?

Probably a bit too subtle for this community, so here's a more earthly addendum taken from William Safire's book On Language.

A West Indian student was having some trouble with oral comprehension in London. One day he asked his English girlfriend what "for having sex" meant. She told him and asked where he heard the phrase.

He said, "I bumped into an English friend I hadn't seen for a while at the pub and he said 'Oh for having sex where have you been all this time.' "