Story lawyer

story lawyer, n. a player who is so immersed in the backstory, metaplot, or history of a specific game world that he/she often attempts to correct the GM or players when they make a gaffe that contradicts the established canon, or forget a detail it. A corruption of “rules lawyer”.
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Addendum While story lawyers are much more accepted than rules lawyers, being one myself sometimes (too much Shadowrun to the head, though I try to catch myself), I can understand how annoying it can get to have someone correct you on the game you're running...