The Letter - S3-E21
Continuity mistake: While George is talking to Nina about the painting, Jerry can be seen taking a bottle of water from the refrigerator. He's then seen holding the bottle when buzzing up Elaine, but then when talking to Nina about the "pop-in", the bottle's gone. It's then back in the next shot when Nina says she's leaving. (00:15:13 - 00:16:00)
The Letter - S3-E21
Character mistake: When Lippman is recounting his conversation with his accountant, he mentions that the accountant, a man, gave the tickets to "her daughter."
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.