Players, tell us of your experiences with GMs! Tell us of the cruelest tricks they've played, and of the sweet revenge you had.
I have played on both sides of the screen, as is the fate of many a GM or would-be GM. As a player, I have caused many GMs to worry endlessly about what I might do to unravel their plans or foil their schemes. As a GM, I am capable of causing abject terror with nothing more than a smile. To begin the thread properly, I shall tell precisely one tale from each side of the screen.
First, as a player.
The setting: 3rd Edition Dungeons and Dragons
The character: A dwarven Fighter/Cleric (level 3-4 at the time)
The adventure: Clean-up duty in a town's sewer system
The challenge: Mostly undead, various low-level monsters
For most of the adventure, it was your usual hack and slash, we split up our rather large party (roughly 10 characters, of which 8 were PCs) to ensure that we did not get flanked, though this caused some of our fights to get rather difficult at times. A few people got level drained by some of the undead, and not everyone had appropriate weapons with which to fight them. However, for the most part we made it through with merely moderate damage, collecting a fair amount of loot along the way.
Then we entered the final chamber. We saw nothing. We were about to give it a once-over to see if there was anything valuable when one of our spellcasters "sensed" something in the darkness...a hatchling shadow dragon. The average level in the party was around 3-4 at the time, though there were a fair number of us. Our most proficient fighter was the half-celestial Paladin (the only such character in the group...his presence was extremely unbalancing, since anything that might give the rest of us a hard time he took out without breaking a sweat), so we decided to let him take the first crack at the beast. None of us had the natural attack bonus he had, at least not without some serious augmentation magic, and he (along with the rest of the group) was benefitting from several magical enhancements at the time. He rolled a 19 on his attack.
And missed.
It looked hopeless. We simply did not have the magical firepower to take down this dragon, and our biggest, strongest, toughest fighter needed a critical hit to even hurt it. On top of that, the dragon was so much faster than us that if we fled and it gave chase, most, if not all of us, would be dead long before we reached the exit. As the only dwarf in the party (and wearing heavy armor at that) running away was simply not an option. If I died, I'd die with my teeth buried in that damned dragon's throat! My mind started racing, looking for possible solutions. Then it came to me: Mayday Tactics (I'll explain this later). During the course of our adventure, we had found a large number of bottles of alchemist's fire. I quickly re-checked the rules concerning grenade-type weapons in the PHB: the attack for a grenade-type weapon was a ranged touch attack, the protection provided by armor was ignored.
The odds were now back in our favor. I informed the others of my discovery and quickly went into action. Everyone started throwing the bottles of alchemist's fire they had picked up over the course of the adventure (we had divided them up evenly beforehand, thankfully), roasting the shadow dragon alive. After roughly three or four rounds of combat, the dragon fled (GM's discretion, the bastard should have been dead after all the damage we did), having done only moderate damage to the rest of the group. Not bad, considering that the GM just randomly rolled for treasure and threw the Shadow Dragon in for kicks...

Now, as the GM.
The setting: Heavy Gear 2nd Edition
The place: Re: my location

The challenge: Finish the campaign
So there we were, meeting for what was to be the last time before classes started up in a week or so. The campaign had been long and quite enjoyable up to this point, but it was getting late. We had already been playing for some time, and having just come to a good stopping point, we debated going on. The action was really beginning to pick up as the climax of the campaign came ever closer. The players were excited, as was I. This was my first time behind the screen where I was able to actually finish any given adventure, despite the loss of one of my players. We wouldn't meet again for at least a month, maybe more, and the players had to go home the next day. I did some quick calculations in my head and went over the remaining campaign notes. At the rate we were going, we could finish the campaign, but we would have to stay up all night in order to do so. I ran it by the players, and they agreed.
Amusingly enough, the campaign (not my own, I was running a pre-gen, as it was my first time GMing the system, plus my first time GMing in a while) had the characters pulling a similar marathon shift. As the game went on, the characters grew more and more tired, as did their players in this perverse form of method-acting. Halfway through they started popping stimulants and analgesics to help keep them aware and numb the pain from the various wounds they started to recieve from the multiple combats they took part in. In the final climax, one of the players actually passed out from his accumulated wounds right in front of the campaign's primary antagonist. Considering that his player had slipped into a Pepsi fueled madness by that point, I count my blessings that he failed that health check.

Anyway, during one of the many fight scenes (one where I had "zoomed out" a bit to increase the size of the playing field, as we were playing with minis and a wet-erase map), I had pitted them against a group of skilled opponents with some quality hardware and roughly decent teamwork. Had this been a straight-out "kill the PCs" fight, the PCs might have had a much, much harder time than they did. As it was, they were defending a specific position, and doing everything they could to keep the PCs from interfering. One of the PCs didn't have any weapons on hand that could reach the bad guys, so he had to use cover (the fight was in a cargo storage facility) to try and sneak around to get within range. Another PC had good range, so he spent most of his time getting behind cover and firing. The last PC was a sniper, and so used the range to his advantage and started taking heads off left and right. Once the shooting stopped, they had racked up a total of nine kills.
There were ten NPCs.
Sniper PC had just nearly gotten his head blown off by an enemy sniper, and the other two PCs had been seriously rattled, especially the ranged one, as the enemy sniper had the Heavy Gear equivilant to a .50 caliber sniper rifle, and had shot THROUGH the cover he was using at the time. They started talking amongst themselves...
"Uh, dude, weren't there ten?"
"Yeah, what happened to the one behind the guy with the Squad Support Weapon?"
"He hasn't moved since this whole fight started."
"Yeah, he hasn't done anything."
"What if he's been trying to aim at us?"
They looked at me, I just smiled. Their guesses were highly amusing to me, since in reality, I had decided that the NPC in question freaked out after his buddy's head got blown apart in the first round of combat, had thrown down his weapon and was hiding, waiting for shooting to end.
"Dude, that guy's gotta be like a sniping god!"
"Look at the smile on the GM's face, man! He's gonna kill us all!"
It was a positive feedback loop. I'd smile, they'd freak out. As they freaked out, my smile got bigger. As my smile got bigger, they'd freak out even more. Finally, they got to where the NPC was supposed to be.
PC 1: "What do I see?"
GM: "There's the body of the SSW guy you guys killed, and it's moving."
PC 1: "I shoot him."
PC 2: "What the hell? There's no undead in Heavy Gear!"
Naturally, the NPC in question was hiding under the corpse of his buddy, hoping that the players wouldn't see him as a threat like that, and that his own friends would have thought he got hit too. After the first shot into the corpse above him, he gave up properly, thus alleviating the fears of my players. Still, watching them try to puzzle out what I had in store was a delicious experience.
