| A rules lawyer is someone who knows all the rules for a game. Typically, rules lawyers are either very experienced at the game, or are people with a neir-eidetic memory who took the time to read the rules.
The stereotypical rules lawyer is one who is continuously nitpicking with the Game Master (GM). While this can be detrimental to a game, the GM is at fault! Really. A GM who can't control the flow of their players shouldn't be running a game. What the GM should do, is set up ground rules. One of the easiest is:
If the GM is breaking a rule, point it out, ONCE. The GM will decide, at that point, which rule - the printed one, or the de facto house rule - will be in effect for the rest of the session. At the end of the session, the GM will research the rule, and determine if the house rule will remain in effect.
In fact, a rules lawyer can be a valuable asset in any sort of roleplaying campaign. The enterprising GM cultivates their knowledge, making them feel a valuable part of the campaign. | 
