My daughter brought me a shiny quarter she got as change from our local Target store. The second I saw how shiny and mirror-like it was, I knew what it had to be. It is a 2005 Oregon
State Quarter, and I got my glasses and checked the reverse, and confirmed; it was an "S" mint mark.
I'm thinking that for some reason someone opened a mint set and spent it.

Is there any other explanation? San Francisco mint coins aren't made for circulation, right?