Factual error: The Devils-Rangers game is at Madison Square Garden, but when the Devils score, a siren goes off. That would only occur in the event of a Rangers goal.
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.