How to screw with the players:
Hand a player a folded piece of paper containing the following instructions: "Read this note. Do not let anyone else see it. Do not tell anyone what this note says. Just say, 'Uh, yeah, okay,' and hand it back."
Adventure: Take a player aside, but close enough to the rest so that the other players can accidentally overhear. Hand the chosen player a note stating, "Ignore what I'm saying right now. This is to mess with (whatever player you feel like tormenting)." Then proceed to conspiratorially tell the player to use his or her most devastating attack on (whatever player you feel like tormenting) when presented with an opportunity, "as explained on the note." The other players—especially the 'targeted' one—will worry about every new scene, waiting for the inevitable set-up. The player you originally pulled aside—if clever enough—will instead wonder about what else is going on in the campaign, and worry about what kind of a sick sadistic bastard of a Game Master you truly are.
Don't say, "You don't find any traps." Instead say, "You don't find the trap." Notice that it's a subtly different sentence. This is especially nasty after you do this a few times… then spring two traps in the same area on them. A corollary to this is for a failed Awareness check. Instead of "You don't notice anything," say, "You won't see this coming."
Describe any setting—nothing is going on, just background dressing. This could be an open field, a deserted street, or the high seas. Ask one of the players, "What's your character's religion again?" Regardless of the answer, or if they question why, just say "Oh nothing, never mind," or mumble something about "… afterlife…"
Randomly roll a die for no apparent reason. Whatever the result, chuckle, giggle, or do the full maniacal laugh, whatever. Don't tell the players what the roll was for, and if they ask what was so funny, 'try' and 'suppress' the laugh again.
Random, pointless 'dungeon dressing' always keeps the players on their toes. They are accustomed to games where things happen, and are emplaced, for a reason. By describing a sudden chill, out-of-the-blue saving throws for no apparent reason, a shadow or movement out of the corner of the eye, or an unexplained noise, set them on edge and make them wonder what's going on. The latter two are especially useful when only one person notices. Are they the lucky one, the one being targeted, or is their character going insane? They'll never know…
Since players are accustomed to everything in a game having a purpose and reason, this can be used against them. They are looking for the out of place thing, and may miss the real clue if it fits into the background too well. For example, if they are looking for the Crown of the Serpent Demons, introduce them to the local King. Describe his Highness—and his powerful magic crown—along these lines: "The King is resplendent in his dark purple robes with gold trim. Rubies encrusting his rings, necklaces, and gold-coiled snake crown. His impressive appearance is matched only by the Queen's beauty, with a scarlet dress, and massive star-sapphire ornamenting her crown." You've just described the object of the player-characters' quest, and all but handed it to them, early in the adventure. Instead, have the King and Queen discuss things not directly relating to the quest—they'll interrupt and change the subject if needed—and send the motley group of miscreants known as player-characters off on a fool's errand looking for a lost item that has long ago been found.
Other pointless dungeon dressing can provide amusement to a twisted Game Master as the players try and figure out the whys & wherefores of an odd item or feature. As examples, stick a teddy bear or other stuffed animal—chained down—in the middle of a weapons rack surrounded by implements of destruction, or have a single shoe in an otherwise empty room. Those players overly accustomed to video games will go mad trying to find out what the item was meant to be used for, assuming automatically that any odd object must have been specially placed by the Game Master for an important use.
Other fun can be had with those players graduating from pixellated games. The best of these is perhaps the missing key scenario. In an apparently abandoned keep, one section—such as the private quarters—is locked and obviously trapped, and beyond the abilities of the player-characters to overcome. Using the logic that the key has to be in a part of the keep they could otherwise get to, otherwise 'the game is unwinnable', they will tear the place apart looking for the key that they 'missed'. It is unlikely that they will think of the real reason they cannot find the key: it's in the pocket of the keep's rightful owner. After all, how often do you take your keys with you when you leave the house? Or do you instead hide all of your keys behind a nested series of locks?
Use this phrase whenever possible, when the characters are out shopping, slogging through the sewers, or even coming home to rest: "As far as you can tell, the room appears to be empty." It'll drive them nuts…
Another paranoia-inducing word, especially when interjected in the middle of a player-character's speaking, is click. There are any number of innocent events that can cause that sound: stepping on a twig, equipment tapping against armour, tired joints popping, or a bug hitting a nearby wall. However, the players don't know what caused the noise, and it could be a trap about to be sprung, an approaching enemy, a sniper setting a crossbow, or any number of horrible events. Let them ponder, but don't really reveal anything. You can hint about possibilities all you want, but never just say, "It was a bug." For added fun, when they are searching and panicking because of a little clicking sound—or they choose to ignore it and move along—then they hear 'click' again. This is especially fun in mid-sentence; it really gets their attention.
Borrow from Dark Dungeons. "The thief, Black Leaf, did not find the poison trap, and I declare her dead." No roll, no chance, nothing. And, "Marcie, get out of here. YOU'RE DEAD! You don't exist any more," when a character dies. The "Knights of the Dinner Table" in Shadis used a variant of this: "As you open the door, the magic-user DIES!" When questioned, B.A. (the Game Master) continued with, "He just sort of choked and then died. I guess it was a trap of some kind."
Have them find a ring in someone's possession. This should be kept in a pocket or pouch, rather than worn. The ring is without blemishes or tarnish, and undecipherable runes are a bonus. By sheer coincidence, the ring happens to perfectly fit whomever first examines the ring. Anything that you can do to hint (without actually stating) that the ring is magical will help out here. Tell the player that it looks like it will fit them exactly. If they say that they will put on the ring, reply with, "Are you sure that's what you want to do?" If they say that they will not wear the ring, reply with, "Are you sure that's what you want to do?" The ring is not magical, but can be sold easily for a good price, as it is a nice piece of jewellery. If the player finally puts on the ring, and is really sure that he or she wants to do so, then respond with a non-committal answer, such as a simple, "Okay." If the player instead decides that the ring is cursed, and throws it away, then remind them that they tossed a valuable item aside the next time they complain about money.
Casually cruel: Nothing says 'Evil Game Master' quite like calmly stacking your dice while describing the tiny pieces that the players' characters are being cut into.
Players rarely, if ever, think to check the authenticity of anything. About the only time that they ever seem to doubt whether something is the genuine article, is when presented with a treasure map for sale. They always suspect those. To use this childlike innocence against them, plant a fake of whatever they are looking for, or optionally, have them simply be in the wrong location. Perhaps when the previous group of adventurers heard the same rumours about the 'Wondrous Scepter of Modius', they planted a replica to help ensure that they would be able to escape to pawn it. Perhaps the real scepter is hidden safely away, and what the player-characters brought back was the phony put on display in case of robbery. Perhaps, after fighting the magical guardians, surviving all of the traps, and braving all the terrors, the scepter they found wasn't Modius's… they were in the wrong hidden temple all along.