The term "Law & Order Syndrome" refers to an actor or actress who appears in a long-running series as a guest star but later appears in the series as a permanent character. The term is derived from the famous television series "Law & Order", where a numerous number of actors/actresses who are currently playing the main detectives/lawyers have appeared in a previous episode several years beforehand as a completely different, temporary character.
Mike: Wait, Chris Noth is a detective now? Wasn't he some random nameless guy in the last season?
John: That's a case of Law & Order Syndrome, Mike.