One of the truths about the Call Of Cthulhu game is that your character is likely to go insane. That did happen fairly often, but only once did the players go insane. I was running Masks of Nyarlathotep, CoC was the only game where I consistently used the modules. I still say that the CoC modules are the best things ever written for any RPG ... ever. In this case they were in Egypt. There was going to be a meeting in a large underground cavern. The group decided that the 5 or 6 of them needed to take on everyone who was going to be at the meeting. This amounted to 70-80 insane cultists, their very powerful leaders and probably a half dozen to a dozen monsters. They knew this going in, despite this the plan started to come into shape.
They had learned a shape shifting spell, they decided that they needed air support, so they transformed one of the PCs into one of the flying monkeys from the Wizard of Oz. That wasn't what drove her insane, it was after they realized that she couldn't fly, and they changed her back that her mind went kablooie. This left them one person less. (oh, and the player wants people to know that she was at work when this happened, and that she would not have let this happen if she had been).
They continued the plan. While they were plotting (oh, and Kevin was right in the center of this) another of the players started furiously scribbling a note. Another of the players looked over her shoulder and mouthed to me "I'm with her, I'm not with them" and went and sat down. Her plan, to be at the entrance of the cave and hose down the leaders with gunfire and then to run before they turned into anything nasty.
The cavern was laid out like this. It was huge inside. At one end there was blank place on the wall, a huge portal of sorts. Toward one end there was a spire of rock, with a stairway going up to it, that the leaders would stand on and do whatever they were going to do. In the middle of the room was a leech pit. I don't remember all the details, but basically what they decided was this ... They brought in a bunch of salt to kill the leeches, they hung someone by ropes from the spire to be a sniper (in such a way so he couldn't get back up by himself) and hung two people by ropes (there is a theme here) down the sides of the leech pits so they could pop up (climbing) and start shooting. They had a couple of shotguns, and a couple of pistols, and some sort of explosives, but not much. The planning went on for several hours. I watched appalled, as did the two gamers who decided to be elsewhere.
I've never had to kill a whole group before, and I didn't this time either, since two of them had some sense, and succeeded in what they wanted to do.
As for the rest, I just couldn't resist. I said "I see no point in spending an hour in which you could be working up new characters, one of the 100 or so of them makes a look roll, and they kill you and eat you." A couple days later the leader of the group said "I have no idea what came over me, that was the dumbest thing I have ever done in all my time gaming". Kevin, to the last time I saw him a couple years later, said it was the MOST ... UNFAIR ... THING ... ANY ... GM ... HAD ... EVER ... DONE... TO ... ANY ... GROUP ... EVER cause they would have won that battle.