Hapended to me a couple weeks ago or so. The moron at the desk explained that the day girl did all the gold and diamond cards, and after they assigned me a room, they gave it to somebody else. Because the card was reswipped my card didn't work, so I went down, told them the card didn't work, she ran it through again, and it worked just fine. (didn't know somebody else had the room yet).
The card worked just fine, somebody said something as I was opening the door, she wasn't happy.
The clerk said she was supposed to have checked the computer to make sure the room I was assigned in the morning was not assigned during the day, she didn't do that, she just printed the receipt, and handed me the key.
Always use all the locks, if they were smart enough to get it right all the time, they wouldn't be manning the desk at a hotel at midnight.