Re: Hilarious Ways to mess with your players.
I have one that I have used. It's a simple approach compared to the rest but it brought me an entire 4 hour session of pure joy.
I made a dungeon where every single door is trapped.
Why was is significant?
Rogue: I check the door for traps
Me: You don't detect any traps on this side of the door.
Sorc: What about he other side?
Fighter: Doesn't matter, he trapped the last five doors, what are the chances that he's trapped this one too. I open the door.
Rogue: I stand way back over there
Me: Okay everyone but [Rogue] roll reflex.
This wasn't just me being a jerk either. The dungeon was a clearly abandoned tower that a Gnome wizard used to occupy and he trapped everything in there (which got him killed). the PCs had been warned constantly by the NPCs in the nearby town that it was a deathtrap and people who wander into it never come back. They just assumed that it was an evil wizard kidnapping people and decided to get in there.
It got to the point where they are too terrified to touch anything. The tower itself was empty other than the traps and one dead Gnome. The tower HAD treasure in it but because I had planned for the party to visit it later in the campaign (along with good enough Search skills to actually FIND most of the traps since it was intended to be pretty obvious that the traps were all over the place to someone with a level appropriate Search skill since I was doing a Girad's Gate sort of thing), I had wing it and had the dead Gnome's family give some level appropriate treasure as a reward for recovering the Wizard's remains.