Character mistake: When Jerry is being questioned on the lie detector test about "Melrose Place", the officer questioning him asks him two control questions that he responds to in full sentences. The questions should have been simple "yes" and "no" questions.
Character mistake: George makes a big deal about wanting to say "I love you" to the girl he's dating and says he's never said it to a girl, just a dog once, and he wants to say it just once. But in s02e01, "The Ex-Girlfriend", George tells Jerry he had to say "I love you" to Marlene when she told him she loves him.
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.