Episode 6.15 The French Mistake
Directed by Charles Beeson. Written by the genius Ben Edlund
Sam and Dean get transported by Balthazar into an alternate universe where they are forced to act like Jared and Jensen, the actors who play them on a show called Supernatural. Starring Jensen as Jensen, Jared as Jared, Gen as Gen, Misha as Misha.
At the Universal Studio..