Continuity mistake: When George is in the boardroom, he places his briefcase on the conference table, and opens it, with the top leaning against a carafe. As George yammers on, the briefcase is closed in a couple of shots, and then open again for the rest of the scene.
Continuity mistake: In the episode 'The Soul Mate,' there is a scene where Kramer is talking to Jerry's girlfriend in The Library with the help of Newman. Kramer elbows Newman through the shelf knocking some books down, but in the next shot we see Newman rising and the books are back on the shelf.
Continuity mistake: George has constructed a model of the boardroom when his briefcase was destroyed. Jerry thinks George should be represented by the Yellow M&M figure. When the camera shows a close up of the figure, Jerry is holding it in a different position than the far away shots.
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.