250. When, at that beginning of a new campaign, one of your player's brings in a type written 30 page background on his new character, and after reading over it carefully you realize that he wants to play an anti-social recluse that has absolutely no desire to be involved in any adventures and will violently resist contact by the other PC's.
251. One of your players brings his girlfriend, a gothic chick whom he continually refers to as 'his b1tch' and he procedes to physically and verbally abuse her.
252. A new player who is the good friend of half your players shows up with a character concept - half-elven vampire werewolf thief/wizard/assassin who is armed with a sphere of annilation and a quiver of arrows of slaying, and you are expected to work his character concept into your gritty low level low magic campaign.
253. You assemble a crack team of reliable roleplayers, who proeceed to create a vivid multi-ethnic party of do gooders, but your adventure requires them to successfully blend into a xenophobic racist culture.
254. You create a devious dungeon filled with traps, only to have the party split up to make exploring 'more efficient'.
255. You spend months creating an elaborate dark campaign setting with an epic story arc, only to have every campaign end on the first night because your players are 'getting into character' and having more fun backstabbing and betraying each other than they are exploring thier environment.
256. You create an adventure that depends on role playing, only to discover that the group of players that advertised themselves as primarily role players is actually primarily hack and slashers.
257. You spend months creating a deep dungeon filled with endless varied monsters, tactical situations, and hordes of treasure for a group that wants 'first edition feel', only to have them complain about the lack of role playing oppurtunities.
258. You create a mystery adventure that depends on problem solving, puzzles, detective work and deductive reasoning for a group of high intellect nerds, only to discover that they there are primarily rules lawyers that are used to solving problems by meta-gaming.
259. You discover that your PC group of close friends primarily wants to meet to socialize and talk about the latest episode of B5/Battlestar Galactica/Firefly/Star Wars/whatever or the release of Starcraft/Diablo 2/World of Warcraft/Age of Empires/etc., and that you are only get an hour or two of actual gaming done in a session, but they get upset with you if you don't continually crack the whip to force people to stay on topic.