Did this to some people at Universal last October.
We checked it our room at Royal Pacific very early the morning we arrived, just before 7am, and were given our card key. The card key worked on the room door, and the handle would turn, but the door wouldn't open. After several tries and pushing, we gave up, gave our bags to luggage services, and went on to the parks. When we came back from the parks the key and handle worked again as before, but this time the door opened. With a little experimentation we found that there must have been someone in the room who had latched the secondary deadbolt on the lock, and possibly the chain. Even with all our pushing and turning I guess they never woke up, or didn't bother seeing who was at the door.