I was going through my change today and came across a 2000 Quarter that was abnormally shiny. I Looked at the mint mark (s) and knew right away that this coin should be in a proof set. Is it common for someone to open up a proof set? Or was this coin accidentally put into circulation? I'm barely a novice at best when it comes to coins. If anyone can shed any light on this coin it would be helpful. I'm guessing this coin would be classified now as a circulated coin? Thanks for any input.