Welcome to flyertalk.

If you wander over to the different hotel forums, there's probably a thread or two in each chains about this happening to people. It's not all that common, but if you travel enough it will probably happen to you at some point. I've been on both sides, where I've opened a door that was occupied (though nobody was in the room at the time) and I've had people trying to get into the room I was in but couldn't because the door was locked.

If you are the one in the room, always make sure to use all the locks just in case. Also there is a good chance your room key will not work anymore once they encode it for somebody else, so check before you go out the next day.