Continuity mistake: After his gig, when Seinfeld is talking to Kramer in the backstage, his T-shirt is all wet around the neck and chest, and a couple of seconds later it appears dry.
Revealing mistake: When Kramer starts rubbing the butter stick over his face, the end of the stick becomes orange from the stage makeup Michael Richards is wearing.
Continuity mistake: When the NBC executives introduce themselves to Bania at the comedy club, their last names have swapped since season 4. Jay introduces them as Jay Chermak and Stu Crespi, when in Season 4, they were Jay Crespi and Stu Chermak.
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.