Oh, man...the question should be "have you ever NOT had this player?"
Three especially troubling troublemakers come to mind.
Troublemaker #1: Pete.
Pete was one of those guys who played RPGs to get away with stuff he couldn't get away with in real life. He'd often do crazy stuff just to see what would happen. Usually Something Bad happened to him, and often to others as well.
Examples:
Barrel, a dwarf barbarian, noticed that a certain pirate captain was wearing a breastplate marked with the insignia of his homeland, the destruction of which was a precursor to him becoming a barbarian in the first place (he was a baby when this happened). I had this whole adventure set up where, after the captain told Barrel where he found the breastplate, the party would go with their friend in search of his long lost homeland.
Instead, Barrel decides that he is going to attack and kill the pirate captain and his entire crew to avenge his people. Never mind that none of the pirates were even born when his homeland was destroyed, never mind that the pirates had just saved the lives of the entire party by carrying them away on their skyship when the tower they were in was overrun by monsters...Barrel, with no warning, up and attacks the nearest pirate moments after the party's rescue. The only other PC who joined him in this fight was the monk, more out of a sense of loyalty to the player than actually wanting to kill anybody.
Barrel's corpse was stripped of all valuables and dumped over the side of the ship, and the rest of the party was made into slaves to replace the two dead crewmembers, at least until the next port when they could recruit actual sailors.
Another example comes from a Vampire: the Masquerade game, where Pete (playing a Malkavian) decides playing ding-dong-ditch'em on a house inhabited by Assamites is a good idea. This is followed by the even better idea of hiding in the bushes right next to the front door of said house right after ringing the doorbell. He was spotted, of course, and the assamite chased our party through the entire neighborhood before catching my character, a 16-year-old Tremere with the combat capability of...well...a 16-year-old Tremere. Even though I had nothing at all to do with it, he poked my eye out with a dagger to teach me a lesson. Granted, these Assamites were semi-outlaws in the city, and we were sent by the Prince to bring them in to swear fealty to him, but still....
Troublemaker #2: Mike
I only played in one campaign with Mike, in which he played a halfling rogue named Charmaine. Charmaine was a big time devotee of Olidammara (sp?), and felt that playing pranks on her fellow PCs was a good way to venerate her deity. Many of these pranks came at inopportune times, such as the time we were fighting dire beetles and she tried to push the archer into the pit filled with the things. It was like playing with an evil kender. She also stole from the party, pocketed things when she thought no one was looking (and almost always blew her Slight of Hand rolls), and tried committing B&E several times in the small town we were using as a base of operations.
Eventually, Mike stopped showing up to the game, and all attempts to contact him proved futile (it turned out later that he owed a large sum of money to an ex-girlfriend who was part of our social circle). The DM NPC'd Charmainee for a while, but once it was apparent that Mike wasn't coming back, the DM had her play a prank on the newest PC, a CN half-orc barbarian of a very unforgiving nature. Charmaine ended up in two pieces, although the LG cleric in the group insisted on raising her, paying for the material components with Charmaine's own money. Needless to say, Charmaine left the party immediately thereafter.
Troublemaker #3: Nate
Nate is another person who I think plays the game just to do crazy stuff that would get him in trouble or killed in real life. His current character, a Khalastar psion named Nishtana, was clumsily shoehorned into the party in the beginning and has yet to really mesh with the group. She is unpredictable, untrustworthy, and violent.
Highlights of Nishtana's adventuring career include:
-kicking in a dungeon door in a secret House Cannith laboratory (Whitehearth, for those of you familiar with the Shadows of the Last War adventure) which had obvious signs of high temperature burns, and it was clear to everyone that Something Bad was trapped inside that room. Despite being told not to, she kicked in the door, releasing a Living Fireball spell that had us trapped in one of the adjacent rooms for an hour until we figured out we could kill it with magic weapons.
-charming, or trying to charm, every NPC she comes across that doesn't cooperate with her 100%. Nishtana has a Cha of 16 and several ranks in Diplomacy (not that you'd ever know it!) and could easily get her way through a simple skill roll, but instead she chooses to use psionics to force others to do things her way. She has succeeded once. She has also tried this on certain party members, also to no avail.
-confronting the head of House Deneith at a party hosted by the group's current patron, (a member of House Cannith), after coming to the conclusion that he was an agent of the Dreaming Dark. She came to this conclusion after the group discovered a jar filled with some kind of psion dust (used to make possession easier for quori spirits) that was being imported by the House as an artifact from Xen'drik. Luckily, the Deneith representitive was surrounded by bodyguards as well as a powerful psion himself, so a fight did not erupt in the middle of the House Cannith ballroom.
-nearly killing the chieftain of a large sahaugin tribe whose island we were stranded on, with whom we were supposed to be negotiating for supplies to repair our crippled submersible ship. Her reasoning was that the 100 or so warriors standing around their chieftain would respect and fear her for proving herself stronger than their chieftain. Luckily for us, we were able to talk her out of it.
Amazingly enough, after the first adventure, Nishtana was actually going to leave the party, and the player was ready to roll up a new character, but for some bizarre reason the other PCs talked her into staying. The devil you know, I suppose.
