Mondegreen
The best Mondegreens are the ones that you don't know are Mondegreens for years.
I think it was only sometime in the last month that I finally figured out that a line in Billy Joel's "Piano Man" isn't quite as unusual as I always thought. I always thought it went like this:
He says, "Son, can you play me a memory?
I'm not really sure how it goes,
But it's sad and it's sweet, and I knew it complete
When I wore a young woman's clothes."
Turns out what he actually wore was a younger man's clothes. Who knew? I always just figured he was a transvestite.