Here's a good one to piss off those impatient types. The PC's come into a room completely empty save an iron door that reads "Only the mild mannered may enter." Anytime one of the PC's attempts to open the door an ethereal giant fist smashes them hard dealing about 2d6 damage or so after a Fort save and sending them flying back across the room. If they hit anyone standing behind them deal 1d6 damage to that respective player and knock them prone. After this occurs the ghostly fist immediately shuts the door resetting the trap. The solution? In order to gain entrance one must knock (politely) on the door and wait for 18 seconds (about 3 rounds) at which point the ethereal fist will open the door and gesture for them to enter before vanishing.
Here was another one I enjoyed (though it really upset my players when they couldn't figure it out). A room is completely empty; No doors, no windows, nothing, save a pedastal with an upright vase. By examining it the PC's find that the vase is filled with water to the brim. If they attempt to leave the room they will find that the door no longer exists and that they are now trapped in this room with no way out. At about this time they'll do one of two things: check for false walls, or knock over the vase. When the vase is tipped over thousands of gallons of water begin to flow from it filling the room quickly with water. In five turns the entire room is filled. In order to escape (and survive) the players must roll a high enough check (you decide the DC) to reveal that the ceiling has a section that is a magical illusion that feels and looks completely real. Once identified that portion of the ceiling vanishes and the rising water carries the party safely into a chamber above them. Go to Comment