The Suicide - S3-E15
Revealing mistake: In this episode when Jerry leaves his room to talk to his neighbour about their fight, when he shuts his door the wall shakes revealing that the place he lives in is not a real apartment.
The Suicide - S3-E15
Continuity mistake: When George and Elaine are at the psychic, Elaine is saying she doesn't know how the psychic can put a cigarette in her mouth. At this point the psychic has the cigarette in her mouth and her hands are clasped. In the next shot, the cigarette is now in her hand.
The Suicide - S3-E15
Other mistake: While George is talking to Elaine and Jerry in Jerry's apartment, Jerry and George talk about how George's mother and father are both bald. We know from later episodes after his parents are introduced that they both have hair.
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.