Oh, another funny albeit slightly mean-spirited trick I played on my players since last this thread died :
So I’m running a game with a player that sometimes pisses me off. The first problem is that he already knows the adventure (and has DM’d it himself) - he assured me when we started that it was some time ago, and that he could dissociate what he knows from what his character knows. Fair enough says I… but in practice his character sometimes has “insights” into where the loot is and so on, or which NPCs to question. Which, over time, can be grating, but I usually re-jigger the scenarios enough that it isn’t too bad and can throw some curveballs at him.
The second and main problem is that he’s a munchkin’s munchkin - in this case, he rolled a character that, in a campaign fairly focused on spying and thievery and ambushes so on, winds up with a passive Perception score in the mid-40s. So, basically, he walks into a room and knows where anything is hidden and there’s no point to there being traps at all as they might as well be painted in neon. Which is not very fun.
So I resolved to punish that.
The group is raiding a very ancient and powerful thieves’ guild’s HQ/treasure house. His “insights” draw them towards the saferoom - which they find empty. He shoots me a quizzical look, I just give him my best enigmatic smile. Then a little later they wind up in a hallway where he spots a fairly badly hidden secret door (that isn’t in the original scenario). He gets giddy because he figures that’s where the new treasure room is, and gets the party rogue to pick the lock. Beyond is a 5x5 room, with a better hidden secret door across from them. “we check that door for traps” he says. “OK. You don’t find any”. They open the door, which leads to… a 5x5 room with a still better hidden secret door across the room. “OK, I open the door”. Repeat a couple times until they are used to the pattern and stop searching for traps. Then the secret door opens… but the previous door immediately shuts down, magically turns into more wall, and a Cloudkill spell bursts from the center of the room. Uh oh. And they’re now in a 5x5 room with no secret door in front of them, but a gigantic metal Fort Knox-style one instead, with a lock that’s just as beefy. The rogue manages to open it, but they still have to spend a couple turns inside the Cloudkill, and no time to check for more traps. And find what else behind the door but a 5x5 room, the door they just took clanks shut and welds itself into the frame, new Cloudkill, new harsh door etc…
All in all they have to go through a semi-spiral of a dozen such gas chambers, and each time the Rogue has to finagle with the locks. They wouldn’t have made it, had the party’s monk (still in the original hallway) not eventually managed to kill the wall and pull them out. At the end of the gauntlet of 5x5 rooms and Cloudkills is a corridor leading to a gold-ish pedestal where a crystal ball rests on a velvet cushion. They go to it (this time checking for, and finding, the last trap, which would have crumbled the whole floor), pick it up - and the ball just goes “Ha. Ha. Ha. Ha.”. Under it is a very old taunting message from the guild’s founder.
I mean, this is a thieves’ guild. They know how thieves think :). The actual loot they’d just thrown down the garden well throughout the years, it’s a simple Climb check to get it back. The group has kept the Taunting Fun Ball, finding it just about the most brilliant magic item ever, and even managed to trick some guards with it later on !

