Generally, most of my bad experiences have been minor, mostly Plots on Rails or Killer GM type things- a huge, ancient Red Dragon in Room #1 of a dungeon being entered by a party of 1st level PCs? Really?
However, a few players stand out for me:
1) The guy who smelled like he was homeless. He wasn't. He was co-owner of the house that he and 2 of the other gamers in the group lived in.
2) The booger-flicker. He thought nothing of digging for gold and flipping the results at the wall. Thank goodness I wasn't the host. Why the host tolerated it, I have no idea.
3) A guy who invariably played royalty as if they were beyond the law- even the laws of other countries- in the style reminiscent of that "He can't talk to me like that" thread here. Many were the NPCs he killed on a whim in various campaigns. Looking back, I worry about that guy.
The most memorable campaign derail I can think of was when a player actually got his PC exiled for openly prosetlyzing a religion counter to the ruler's. The player and the DM didn't see eye to eye on why this happened, or even that it could or should happen, and after an hour or so of argument, the campaign ended.
Fortunately, not only were they good role-players, they were good buddies and we all still game together with no hard feelings. Just not in that campaign anymore.
***
Another city, another group, another game, another time- I was in a supers campaign, and the party was going out to confront a particularly dangerous group of villains, and we knew we'd be outnumbered. We managed to find 2 mercenary snipers to provide fire support, and went on the mission.
This apparently was cause for the GM to believe that they would make the scenario unchallenging to the PCs because...
In the hours before the confrontation, the 2 professional snipers were unable to find a single location in the chosen encounter area- full of tall trees and buildings- from which to take a shot. As a result, they never did.
It was a 20 minute (game time) melee, which the PCs just managed to survive. On the plus side, our hired snipers didn't get hurt (or even shot at), nor did they expend any of their expensive ammo, while pocketing their full checks!
We didn't play another game with that guy behind the GM's screen.