I found a 2004-S proof set nickle in my change Friday night when emptying my pockets. :D
Gotta love it when kids get fancy coin sets.
Proof coins are the ones that are specially struck at the mint and placed in protective holders. They are never touched by human hands - well, until the kids crack 'em open and spend them. They are easy to spot since the backgrounds are VERY shiny and the foreground items are frosted in appearance.
RJ