When you are having sex with a girl during a lightning storm and then there is a flash of lighning and during the split second that you cant see the girls brother pops out of the closet and switches places with her so when you regain your sight you realize you are actually fucking the girls brother.
Also known as "The Ol' Switch-a-roo"
Tom: Dude you will not believe what happened to me the other day.
Tom: Well I was with this girl at my place and we got into it and then there was a flash of lightning and she pulled A Larry Bird!
Jack: My god...