Continuity mistake: Kramer just gave Elaine the bench she wanted. The Apartment door is open. Cut to Jerry who says all salty "Well, goody for you!", and the door is closed. It is open again next time it is in frame. (00:17:40)
Continuity mistake: At the beginning of the episode Jerry tells Elaine that he vaguely remembers "doing something"; Julia Louis-Dreyfus puts the remote down and brings her hand up twice in two separate shots. (00:01:30)
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.