My worst campaignas a player:
A friend of mine had this idea to play some homebrewed western D&D world of his.
In short words you can describe the world as "western with magic and a little steampunk" and you could soon see that his idea was not very well planned and thought through.
We were escorting a wagon trail to a town without train connection (or so) when we where attacked by a quite large indian "war pack".
Due to a little roll luck we saw them early enough to position the wagons in a circle and get a lot of cover.
The war pack startet orbiting around the wagons and tried to shoot us with bows and guns.
Since we has pretty good cover the GM was not able to effectively hit us but we one by one shot his NPCs. Up to the point where he got upset and moved them just as far away (more than 200 meters) that we couldn't hit them well enough anymore because of the range modifier on the attack rolls.
Then he startet to describe how we could see them preparing something and how we presumed they where performing some ritual dance.
Remember: Both side - us and the war pack - where to far away from eachother to effectively target and hit with ranged weapons.
Nevertheless the GM did not have a problem to send a big "fireball" to towards our wagon camp, which was so precisely aimed that it could exactly hit us PCs _and_ strangely fast set us on fire.
My answer was something like "Well, if you're really sure about this..." with a clear undertone of irony. After that I took a little nap until the gaming session ended earlier than usual.

as a gamemaster:
The rule system was Earthdawn.
The setting a little down grown out of an old caer (the magic vaults from Earthdawn) which lost contact to everything that took the trading route to the north (east, west and south were okay).
The PC party rallied up and recieved a job from the town's mayor to investigate the problems north.
Result: They found a contaminated area surrounding the entrace to another old caer. Presumably there was a demon problem inside.
Over the time of the campaign several characters died.
Sometimes out of bad luck. Other times out of stupid player decissions.

And I can be a nasty GM when players are constantly stupid

But that was not the main problem of this group.
One of their problems was that they never listened to me when I reminded them about their karma points (to get extra dice on main roles) or when I told them the 10th time the house rule xy from one of their older PnP rounds was not applied in the current round (which was in their favor since the mentioned house rule was a disadvantage to the PCs).
another problem was that I demanded player initiative at least from time to time.
Sometimes they they were good at this but most of the time not really.
One of the outstanding player actions was when one of the female players stated that her character (a troll airpirate or warrior. i don't remember) went jogging arround the inner side of the city wall to loose some weight. Just because she couldn't "handle" the ingame social PC-NPC-interaction and I didn't agree to simply give her some enemies to slay. (side note: the player itself was on a diet at that time

)
Another time they were exploring the contaminated caer and had already fought some smaller worm-like abberations. They rested in one of the abbadoned rooms from the last fight when I started to describe that they could hear something in the distance. This time it sounded bigger than the other worms.
For one short moment I thought to see a glimps of cleverness in the players minds and they would flee as it would be the wisest in the given situation. But as often they discussed far to long about their options and thus lost the option to flee and instead hid themselves in a dead end.
Okay. I said to myself the "big bad worm" is not so smart. If they stay calm everything will be good.
But the party convinced me I was wrong before about this glimps and started to buff themselves up with all magic they had available.
Bad idea.
Demons and demonic constructs in Earthdawn are often if not always attracted by magic. The more the better for them.
So instead of the PCs staying low profile they blinded the big bad enemy with their magic spotlight

and in the end 3 of 4 PCs where dead.
At this point the campaign was still funny for me as the GM but this changed in the further sessions.
The PCs more and more lost themselves in discussing what they could do to proceed in the plot but only seldom clearly said what their characters really do. And instead of helping the endangered town they travelled around and even considered to completely leave the area. Only to find the contaminated area massively grown and the town under siege when they returned weeks later.
Again they discussed a lot of things but did very few.
Even me reminding them that they could do almost everything but have to decide on a few clear things to really execute them did not help.
In the end they wre upset and porjected this feeling into the game by playing jokes with me and the ingame world.
They joked about putting some announcement on the cities black board to lure out some mid-size worms that had been able to sneak into the city some nights before (more or less due to the players fault

).
The announcement was roughly: "The worms won in lotto. Please report to us to recieve your prize."
Next morning the PCs went to get some breakfast when they crossed the marketplace and found a big crowd of people exitedly questioning the mayor for whom this "lotto" was and who did win now and what did they win.
The players teased the people a lot and soon found themselves confronted by an angry mob.
A couple of other unwise decisions and ruel discussions later 2 of 4 PCs had been sacrificed by the mob to a big demon worm and another PC had been killed by the town's militia.
Few weeks later I was blamed by at least one of the players for the bad campaign result ("You did send us far to strong opponents!") and soon one of the other players will start as the new GM in a new campaign.
I'm looking forward to the result ... with a little sarcasm.
