Continuity mistake: In one scene George is solving a Rubik's cube in Jerry's apartment. When he leaves, he places the cube on Jerry's kitchen counter, and it is in the middle of the counter with the red color facing out. In the very next shot, it is moved much closer to the far edge of the counter and the color facing out has changed.
Continuity mistake: After George and Jerry discuss the fact that George will never have sex again, they stand up to leave the coffee shop and George places a large book under his arm. In the very next shot, he is holding the book in both his hands.
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.