This happened to me a couple of months ago, at the Hyatt Regency in Crystal City. I had checked the hotel's website just before placing my Priceline bid, and it was showing sold out.

When I got to the hotel it was jammed with members of a motorcycle "club" who were going on a big organized ride the next morning. My guess is that the hotel, anticipating many no-shows, probably oversold on purpose (I ended up getting upgraded to a huge room that was essentially a junior suite).