I've faced alot of terrible people.
The first 'bad' play expeience I had was at a Warhammer tournement, back in the days of 5th Edition. In my first game, I ended up against a Khonate Deamon army. Now, I was fairly new to the hobby at this time, so under normal cirumstances I would have been a bit edgey to face this army.
Except my opponent was nine years old using an army his daddy had painted for him.
Throughout then entire game, he whined and was a poor sportsman. When my Grail Knights charged his Bloodletters, I asked him, "Ok, what's the WS on you Bloodthirster?"
"Oh, it's 10."
I said the name wrong, so he tried to claim his unit had WS 10. The game contiuned like this for 4 turns until it bacame a total mosh pit. Finally, I got my chance, and cast the old High Magic Spell, "Banishment" Naturally, he was very unhappy, as this little move took out most of his army, costing him the game.
He started to bawl, and his father came over asking me to throw the game and let him win. When I refused, he went to the GM to petetion this. Nothing doing, I'd won. I went on to win second place over all. The kid went on to get kicked out of the tournement after he pulled the same stunt on another person.
Then, there is my friend, who is the only 40k player I really have games with often. He plays Vanilla Marines, and constantly is trying to bend the rules. He crys "Cheese" constantly, even though he has tried to shoot a lascannon on a Landspeeder through a 1 cm wide window (that had was 2 feet away from) at a Vyper that had moved 12" in the last turn. I can't stand him. Not only does he question EVERY RULE, resulting in huge arguements in the middle of the game, when I show him my models, he looks for the tinest flaw, that one little splosh that is in the least visible part of the model. Meanwhile, he primes his marines in black and gives them one layer of white paint on certian parts, and leaves his bases black, yet he still believes his painting is superior to mine.