Continuity mistake: When George is lying on the floor in his underwear, a newspaper is on his right side. When they take the reverse shot (Jerry's view of him lying there), the newspaper is on George's left side.
Continuity mistake: In the opening scene in the locker room, George's shoe goes from off his foot to on to off to on.
Factual error: Newman says that the "Magic Loogie" incident happened on June 14, 1987, when the New York Mets lost to the Philadelphia Phillies due to an error by Mets first baseman Keith Hernandez. However, the Mets played the Pittsburgh Pirates on that date, and the Mets won 7-3.
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.