Factual error: In the scene where Anna is standing with George beside his father's GTO, Elaine suddenly runs in and pries her away from George. Elaine and Anna get into the cab. As the cab pulls away, the side door is marked as an NYC cab with a taxi license number, etc., but the license plate in the back is a California plate.
Continuity mistake: In the episode "The Suicide", Kramer discusses suicide with Jerry, and says that he'd give someone 72 hours to come out of a coma before all his property is up for grabs. However, in the episode "The Comeback", Kramer tells Jerry that he never knew that people could come out of a coma.
Other mistake: The tenant board member tells George that Mr. Eldrige in apartment 8C has decided he wants the new apartment. However, the new apartment in question has a 3-digit number - 808 I believe. This indicates that the building would not have single digit numbers and letters (Such as 5E, 8C, etc.) as part of its numbering system.
Continuity mistake: Towards the end of the episode, it transitions between the Yankee ball at night and "Susie's" wake which is during the day.
Revealing mistake: When George puts down the hairdryer next to the picture of his "fiance", if you look at the bottom right of the screen, you will notice a blue flame most likely from a torch the crew used to create The Fire.
Continuity mistake: When Kramer spills the tomato juice all over Jerry's walls, he grabs a light colored oven mitt to wipe it off. In one shot he wrings out the sopping mitt over the juice pitcher. When the shot changes and he tosses the mitt back onto the counter, the mitt is still the original color with no hint of red stains.
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.
The Fatigues - S8-E6
Continuity mistake: At the start of the episode, Cynthia is drinking a glass of wine. While speaking near the end of the scene, she is holding the glass in her right hand and says, "You know, guides you in your career path." The next shot is of Jerry and instantly the glass has disappeared from her hand with her right hand resting on the table. You can also see the glass is closer to her left hand now.
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.
Character mistake: When George tells Jerry about his date with Anna, he says he is picking her up in his father's 1968 GTO, but the car he is leaning on in the park is a 1967 GTO, which has a completely different body style.
Continuity mistake: When Celia breaks out of prison and comes to George's apartment, George opens the door and starts talking with her. After George says "And you decided to pop in" the shot shows Celia but the door has now closed behind her. It would have been impossible to close without us seeing it because Celia would have had to have moved forward to close the door.
Continuity mistake: Due to the effect of the red glow on Kramer's eyes, he confuses the pitcher of tomato juice from Jerry's refrigerator for a pitcher of milk, pours it into the cereal, then gags and promptly tosses the contents of the bowl into the trash beside the fridge. In the close-up he tries to wipe some of it off the wall, but there's still a huge red mess left on the wall, in the trash, etc. In the next wide shot of Jerry and Kramer, the trash and wall are spotless.
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.
Continuity mistake: In the final scene where Newman is going to eat the bag of muffin stumps, Newman removes the bottles of milk and then takes a bite out of his muffin. He then takes a drink from the glass but he never filled the glass with milk in the first place.
Continuity mistake: In one scene George is solving a Rubik's cube in Jerry's apartment. When he leaves, he places the cube on Jerry's kitchen counter, and it is in the middle of the counter with the red color facing out. In the very next shot, it is moved much closer to the far edge of the counter and the color facing out has changed.
Continuity mistake: When Newman is first eating the chicken in Seinfeld's apartment, the amount of meat on the leg changes without Newman putting down the leg and getting another one out of the box.
The Checks - S8-E7
Continuity mistake: When Jerry is showing George how he twirls the umbrella in his apartment, it has distinctive bright red and yellow stripes. He sets it on The Couch and then Kramer accidentally sits on it. It is now dark colored.
Continuity mistake: We find out George's parents sleep in separate beds. His father says "Thirty years ago we came to an agreement. It was the only way I could get some rest. Estelle's got The Jimmy arms." This contradicts S5E10, "The Cigar Store Indian", when George's parents come back from their trip, Estelle finds a condom wrapper in "our bed." Also Frank asks George, "You were having sex on our bed?", and Estelle follows by saying, "Who told you you could have sex in our bed?!" (00:16:50)
Continuity mistake: When Morty's exercising first wakes Kramer up, his clock reads 5:30. Then in another shot, the clock reads 5:26. (00:10:05)
Continuity mistake: The exterior of the video store in the episode looks different from the interior. From the inside, we see a neon "open" sign on the window and other writing, and also no exit sign on the door, all of which is contrary to the outside view of the store. (00:09:25)
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.