We've all heard the stories about That Guy. He's the killer DM who revels in TPKs and devising entire dungeons whose sole purpose is to obliterate the player characters with no real hope of either escape or-- perish the thought-- success.
He's the player who tells the other player characters that he's an elf fighter-thief, when he's really an assassin, and will slay the other party members when they're at their weakest to take all the treasure (and, incidentally, end the campaign in the process).
He's the person who will have forgotten his wallet when the pizza arrives, but brings a 6 oz. bag of popcorn when it's his turn to pony up to bring food for the game. Oh, and can you give him a ride to the game?
He's the guy who, if a woman player shows up, immediately starts either a) being completely obsequious in a transparent attempt to win a fleeting smile from her, or b) being completely rude and obnoxious, making it very clear that he thinks that RPGs are a "man's game" and that any female who deigns to participate is fair game for being hit on in the way most certain to make everyone else at the table uncomfortable.
He's the guy who has done a statistical analysis of all the weapons in the rulebook, and has figured out that if one is using a glaive guisarme, that has the best damage-to-g.p. cost ratio of any weapon in the book, as long as the DM is using weapon vs. armor class. And if the DM isn't, he'll insist that that rule be used, because otherwise "it wouldn't be fair, because I chose this weapon specifically because the rules say it should be better against splinted mail."
He plays a female character, and acts in completely slutty ways and engages in outrageous behavior that no actual female would ever even consider, even if she was a crack whore in desperate need of her next fix.
He insists on arcane and incredibly complex treasure allocation rules, based not on the use that any individual character can put a given magic item to, but rather who has X number of items, and who is now "due" to have an item, and woe betide the person who trades an item of unequal x.p. value!
Any time the DM says "well, because you're attacking the ogre, nobody else can, because you're blocking the doorway", he argues for a full 30 minutes that what he really meant to say was that he draws the ogre into the room, so that all the other party members can also attack in the same round. Because, it wouldn't be fair if his character was the only one that could get hit.
He insists on playing a ninja, even though it's a game set in an analogue of Medieval Europe. Every time. Every... frigging... time.
He insists on playing a drow ranger wielding two weapons, even through the campaign has nothing to do with the Underdark and drow would be completely out of place in the setting. Every... frigging... time.
But you know... I never played with That Guy. Not once, in 35 years. Everybody else seemed to keep playing with That Guy, so it came to be a given that That Guy was playing everywhere, and was infecting every game, but I managed to escape unscathed.
And then people started to write games whose rules were designed to thwart That Guy. We started having formulae with "challenge levels" and so forth, so that every dungeon and every encounter therein was perfectly mathematically balanced according to the rules. We had rules that required that the exact location of each character in every encounter was tracked within five feet, lest someone think they were somewhere and the DM think they were 10' away, and "DM fiat" be required to resolve the issue.
I must be blessed. Because I never played in a game with That Guy, and I never felt the need to have rules to counteract his diabolical effects. But in all seriousness, am I really that unique? Have I just managed to wander the paths and byways of RPGs for years and years without encountering the legendary RPG Troll? I'm beginning to think That Guy is a myth.