Avenging Wallflower. These players play characters with a vast amount of unheralded, unseen power and go out of their way to be meek and unassuming, at least until a dramatic confrontation occurs in their vicinity. Whenever active, outgoing characters do something interesting near them, they will feel an unholy compulsion to leap into action, screaming and blowing things up with their incredible
powers, simply to interfere in the process and thereby feel that they've done something. It's quite common in Anime, and I've seen about a thousand 'I'm a dragon ploymorphed into a halfling'.
Axebeard Law. In fantasy games, all dwarves should have the words "axe" or "beard" somewhere in their names.
Dark Lord. Stereotypical villain in most fantasy games and any other game with a fantastical bent. Dark, scary, and obviously seeks the complete subjugation of everything. In the earliest games, these guys rarely had any motive beyond "well, he's evil - you need more?"
Deck Of Many Things Law. In Dungeons & Dragons games, players will always screw around with a Deck Of Many Things if they find one.
Facial Hair Law. The style and amount of facial hair on any character will indicate alignment and general tendencies: goatees are either evil or poseurs, full beards are lovable big guys, long beards indicate wisdom, and scraggly, unkempt beards mean insanity.
Grudge Monster. Non-planned monster/adversary a gamemaster secretly puts into the adventure after the players piss him off.
Heroic Fortitude Law. When wars occur in fantasy games, the heroic side typically loses almost every battle, but will somehow win the war.
Herzog's Law. Given a choice between gaming and dating, many gamers would be surprised that they actually have a choice
Illiteracy Law. RPG books always have far more typos than any other type of publication known to man....
Intervention Of Reality Rule. D&D-based novels do not in any way take into account that powerful D&D characters can survive massive amounts of damage without blinking.
Monkey's Paw Rule. When players get wishes, the gamemaster will make every attempt to pervert the wording of the wish into something harmful (usually by interpreting the wish as literally as possible). Legendarily true in D&D games
Mook Law . Any NPC who the players join with and the DM doesn't bother to name is an NPC that invariably dies.
Munchkin. Player whose goal in the game is to amass as much power and kills as possible, whatever the costs to role-playing, the storyline, fairness, or logic....
PineSol Law. In fantasy games, all elves should use foliage types or some reference to the sun in their names.
Railroading. Any time the gamemaster will not allow players to deviate from the adventure's one set path or even make their own decisions.
Roll-Playing. When character statistics and rolling dice (especially for combat) become more important than role-playing or telling a story.
Saturation Law. At any given point, at least half of all gamers have plans or dreams of creating and publishing their own RPG.
Seagalism. When a player attempts to play the same character type and personality in every game they play. (So named for Steven Seagal, who plays the same character in every movie.)
Stoic Moron Law. Unless forced by the game rules and the gamemaster specifically tells them they're afraid, most players will assume their characters are fearless and have absolutely no problem doing things like running through a tunnel full of tarantulas or sticking a dagger into a towering, screaming monster made of decaying flesh, twisted metal, and half-consumed victims.
Tavern Rule. In fantasy games, player characters usually not only start the campaign in a tavern or inn, but immediately become best friends
Trains On Time Rule. Any government overtly or secretly controlled by an evil force no longer has to worry about bureaucracies, internal politicking, citizen oversight laws, logistics, or budgets.
T-Rex On The Plains. A particularly irritating form of Railroading where the gamemaster uses huge, nasty monsters (or high-level adversaries) to scare players back onto the path
Vacuum-Packed Dungeon Law. All high fantasy games contain underground complexes that no one built, full of monsters that need no food and never leave their assigned room or corridor.