I feel bad. The most evil thing I've done is to have a boss set an ambush after the party decided to camp in his soldiers, barracks after they murdered some of them!

In all seriousness, I have this campaign going on where the PCs are going against this cult that has people being mind controlled and getting mutations from a type of green crystals. Long story short, my PCs intercept a shipment from a mine and the rogue takes some small crystal shards and puts them in his coin purse.

Right after they learn of the mind control thing, I make a "ohhh right" face, and give him a piece of paper (which I rarely use, should use them more, they're awesome): "Where do you keep the green crystals you got at the mine."

He makes that "oh ****" face and sends me back "in my coin purse".

Now keep in mind, the other players are all silent, just staring at those papers being scribbled and sent back and forth.

My next one: "Roll a will save. Don't tell anyone why you're rolling, or what kind of roll you're making."

He takes out a d20, rolls it, nobody knows why. Rolls a 1. The guy's fair though. Plays his character as if nothing happened.

Then, the next session, they're still in town, I have a shady-looking man walk by them, growls at everyone, EXCEPT the rogue. They followed him for half of the session.