Lawful Good Rules Lawyer

Lawful Good Rules Lawyer, n. a player who informs the GM of game rules that would be detrimental to the player or the party; see rules lawyer, alignment. [FSG]
GM: "Okay Sara, your critical hit will destroy the vampire." 
Sara: "Um...undead are immune to critical hits. He only takes a third of that damage." 
GM: "Thanks for reminding me! The vampire turns into mist and disappears behind the crack under the door." 
Bob: "Why the hell did you tell him that for?! Whose side are you on anyway?!" <Br>
Sara: "I'm on the side of truth. What side are you on?"
'Bob: "%@#$ Lawful Good Rules Lawyer!"