It doesn't matter if your live group is composed by the most innocent people in the world. Someone will always play The Hidden Evil Sadistic Thief Backstabbing Scheme-making Bastard.

At the tabletop games, someone somewhere will think it's a good idea to bring their not-quite-stable friend/girlfriend or boyfriend/spouse/family member/stranger to the game. They will ruin it for everyone by making the Mazes and Monsters mistake of not realizing it's only a game*.

*I've only killed one fellow PC ever in my experience with tabletop/PbP. Lucky for me, I found out afterward that the player was a paranoid schizophrenic. He did not take it well.

When it comes time to make characters, if one character is playing a paladin, then there is a 73% chance that another party member will want to play a necromancer.

When it comes time for character creation, one player will almost always misunderstand some aspect of the rules (+19 LA...in a level six game...no, dropping half-titan will not fix the problem)...

If player versus player combat is stated to be likely, few party members will willingly share details about their capabilities, even if it would be to their benefit.

MW Murphy's Law #30: Whenever a situation arises which can only be handled by one party member, the player responsible for that party member will stop posting for an extended period of time.

MW Murphy's Law #31: When collecting loot from a dungeon, or after winning a difficult battle, at least half of the awarded items will be set aside, ignored, and sold at the first opportunity. The most useful or most powerful item in the bunch will always be one of the rejects.

MW Murphy's Law #32: Characters with below-average intelligence or wisdom (or both) will always fall into one of two categories: MacGyver, or Leeroy Jenkins. Either the character will conveniently ignore the fact that he shouldn't be smart enough to think up the brilliant strategies he constantly uses, or the character will intentionally use his stupidity as an excuse to derail the plot and endanger his allies.

MW Murphy's Law #33: When recruiting new players to a game thread, at least one of those accepted will always be the GM's least favorite type of player (powergamer, essay writer, clown, rules lawyer, etc.) No known screening method is capable of preventing this.

#34 Settings dubbed 'realistic' never are.

#35 Many people consider emo to be realistic. They will never let you tell them how wrong they are.

#36 There is no greater trainwreck than a socially inept/immature PC trying to play a character with high charisma.

MW Murphy's Law #37: Whenever you get a good group of players, one of them decides to play a murderous-kind of character that doesn't get well with the others.

MW Murphy's Law #38: Whenever you finally find or settle on a good GM, he'll almost immediately spend a full day utterly silent, leaving you gnawing at your fingernails wondering what is going to happen.

Edit: Py4ne, you should really compile these in the OP.

MW Murphy's Law #39: If a potential player posts in your ad with excessive amounts of exuberance and excitement about your game; they will be vapor before the game actually gets started.

EDIT: ...for the record thequietone has been totally nailing it with regards to these laws; I've experienced about 99% of what he's posted here.

Given the choice between talking their way out of a situation or blowing up everything in sight with magic and explosives. The players will always choose the latter.
-Shane

41. If you do an excellent job of roleplaying your bluff/diplomacy/deception/persuasion check, you will roll a one.

41a. If you just say "I deceive him!" you will roll a 20.