I've been given an occupied room. I took a late afternoon flight LAX-IAD, arriving past midnight at IAD (but still before my bedtime). I'd done online check-in, and someone at the front desk messed things up enough that the key they handed me (which had my name on the envelope) went to an occupied room. Fortunately the occupant had flipped the slide catch so I didn't get in. Back at the front desk it then took about a half hour to get sorted out because the computer system was in the middle of some kind of forced update.