The Library - S3-E5
Continuity mistake: When Bookman is lecturing Jerry in his apartment, he picks up a navy book with gold lettering and places it on the bookshelf in front of a pink book. For the rest of the shots, the navy book disappears and the pink book is back in front.
The Library - S3-E5
Character mistake: When Marion mentions Mr Bookman she says he'd been on the job for 25 years. Yet later on he tells Jerry he's been on the job since 1971, which in 1991 makes it 20 years.
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.