For 20 years I did a lot of traveling on company business, and can say this type of thing happens more often than you might think. I've checked into about every major chain of hotel you can think of, and getting keys that don't work, walking into rooms that weren't cleaned, or rooms that are still occupied, does happen.
Worse I ever had was after a very late, delayed flight, I got to my hotel at about 2:00 am, unlocked the door and walked in, and there was someone already sleeping in the bed.
On another occassion, while staying several days at a major chain, I came back to the room after a long day only to find the door not completely shut. Housekeeping had turned the deadbolt with the door open so they could go in and out (that holds the door about 1" open), and never closed it when done. My room door stayed that way all day. I checked and nothing was missing, but that did get a large complaint to management.
For DVC, after 12 years, I can only recall one time we got a room that had not been cleaned yet. It did make me appreciate housekeeping though. There were cookie crumbs all over the floor, and crayon marks on the walls.
All I can say is Que Sera (sp?), people are human and mistakes will be made.