Revealing mistake: When George puts down the hairdryer next to the picture of his "fiance", if you look at the bottom right of the screen, you will notice a blue flame most likely from a torch the crew used to create The Fire.
Revealing mistake: When Kramer and his lawyer, Jackie, are in the taxi; when Jackie says "who told you to have a pow wow", a man is seen walking across the rear window. The taxi is supposed to be moving, but the man's position stays the same (the taxi should be pulling away from him). The actual traffic filmed for the green screen doesn't seem to react to a man jaywalking in front of them either, as it was someone walking across the set.
Chosen answer: George does mention his brother in the episode "The Parking Space". They actually really never mention anything that contradicts the fact he had a brother though, it just isn't mentioned.