Too many to count honestly, I'll put up some of my favorite *palm forehead* moments on the GM side of things. 3.5 D&D, quick one time dungeon as a training session for new players as well as a get to know you for the entire group. Two rogues, one monk, are the primary culprits with this (there were other players, but these three were the ones that really made the magic happen). They get into a room that has one, I'll say that again, ONE, dire bear. The monk's response? I can run faster than it, and am fast enough to dodge anything that it can hit me with, so I'm just going to run it in circles while you (the rogues) try to snipe it with arrows, and even tries to convince the wizard that he can reflex save out of a fireball or whatever else to feel free to AOE on top of him. Before we are able to stop him, he jumps in and starts running the bear around in circles. This results in one of the most amazing series of failed rolls that I have ever been witness to. Initiative - the rogues are ahead of the rest of the group by a bunch, they're going first. First rogue, readies his bow, takes a shot, rolls a 1. Makes a few other rolls, drops his bow. Second rogue, readies her bow, drops her bow as well. The group doesn't do anything, laughs at them, and the bear keeps chasing our poor monk. After another round, they've got their weapons back in hand and are ready to do some damage. They do a grand total of about 2 damage because one of them misses entirely. By this point the rest of the group is laughing so hard that they skip their turns and just admit that they're doing nothing for the rest of this fight to let the rogues try to kill the bear all on their own, leaving the monk stuck running in circles as long as it takes. Long story short- it took far longer than ever needed, including a few arrows that hit the monk, to finally kill the one dire bear. Same campaign, same female rogue that couldn't hit the dire bear. After a bit of investigation the players sneaked near the enemy's main camp in hopes of overhearing their plans. The rogues snuck in, and managed to catch a glimpse of the leader of the army. They came back to the group with the information and the group decided that it was 'worth a shot' to try to snipe him. For the rest of the session the group came up with a list of spells, items, and gear that they were going to use on the one rogue in the attempt to get her to hit with sneak attack damage. There were blessings, prayers, potions, enchantments, it was over the top crazy. I mean, it has to be some of the best team work that I have ever seen from my players, EVER. I figured out the roll needed, and it turned out to be something really great, even at the distance they were working with, like needing to roll a 4 or higher to hit and it honestly made me afraid that my campaign would have yet another weird twist to it because if the leader got taken out by a well rolled sneak attack I had to rewrite and redo a lot of work. Luckily for me, she dropped her bow again and gained the attention of the entire army. With no coordination the party split up running in random directions to not have to deal with the entire camp of soldiers hunting them. For the next session or two the entire game was running, hiding, and the players trying to find each other. From that game on my players have always had a meeting spot (usually back at the city at a tavern or shop) just in case they ever get split up again. Last one, I promise. Modern setting, high rise building in a city. Had all 60+ floors mapped out and ready to play through. The party started on floor 50-ish, and they know that the police were starting on floor 1 and working their way up to find them (they made some really bad choices and got caught in a building wide lock down to find them). I had it all ready for them to do what they had done up to that point and fight their way out. Everything was pointing to them going all in, and making it a blood bath. They go down a few levels, turn around, and start going up to the roof. In a series of amazingly bad ideas they shot out a window of a neighboring building, had one of them JUMP from the roof of the first building into the second building (amazing roll that actually worked while I was expecting it to be more of a suicide jump), and then shot a rope from the roof to the room that the player was standing in so that he could tie it off and make a zip line for the rest of the group. This was 600+ feet in the air, and all of the attention from the cops were on the entrances and exits, so they made it across the street and out of the building before anyone could report what had happened and the cops could react. 60 floors of a high rise building kindly mapped out with all of the cops they'd have to kill to get out the front door was quickly moved to the 'unused maps' section of my binder and we had to skip the next two scheduled sessions because of how behind their stunt put me.