Yet another trap in the arsenal of sadistic and snarky pitfalls. This one is simple, but it definitely helps if your players are used to just walking around, kicking down doors without much regard for who or what is on the other side.
The knock trap is triggered when a character tries to open the door. The door opens a crack, but stops abruptly as a glowing arm reaches out, striking the offending character for *insert level appropriate hit points here* and knocking them on the ground. It then waves its finger in a "no-no, shame on you" fashion and returns behind the door, slamming it shut again.
A disable device check can deactivate the trigger, but the best way to bypass the trap is to simply knock on the door. Once a character knocks, the door opens a crack again, but this time the hand waves for the character to come forward and the door creaks open on its own.
A fun one if you don't have any rogues (or folks who can disable traps) in your party, but still want to give them a trap they can bypass. My players went knock-happy after this one though, making sure to announce themselves when confronted with just about every locked door after this one. Made for some interesting encounters.