Looking for Good Ideas for a Prank?

A friend of mine recently pulled an embarring prank on me -- I need to get some serious revenge. My boyfriend and his roommates are having a keg this friday. This friend of mine will be there and he will be getting very drunk. I think this would be the perfect situation to get him back but im having trouble coming up with ideas. Anyone got an idea? I really want to get him good. Nothing is off limits. Hit me with your best stuff