The Letter - S3-E21
Character mistake: When Lippman is recounting his conversation with his accountant, he mentions that the accountant, a man, gave the tickets to "her daughter."
Revealing mistake: In the sequence when he gets shot by the FBI agents for stealing cable, Jerry Seinfeld is wearing ear buds, quite visible when he collapses by the door. Evidently the stage props even loaded with blanks were too loud for his taste. (00:09:00)
Continuity mistake: The scene which George arrives at Beth's apartment in a messy condition to admit his remark was in jest, his shirt's hem and collar change between shots. During this scene, especially when he talks to Jerry on the phone, his collar is up and down; his hem is in and out of his trousers.
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.
The Doodle - S6-E20
Continuity mistake: In this episode, Newman's apartment number is 5F but in all the episodes both before and after, The Apartment number is 5E.
Continuity mistake: When Kramer is on the Regis and Kathie Lee show, after he puts up the cofee table, Kathie Lee has her coffee mug in her hand. In the next shot, her hands are empty, and in the next shot it's back in her hand again.
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: Right after Jon Voight enters the taxi, Kramer sticks his arm in through the cab's rear side window, where Jon Voight was sitting. Jon Voight grabs the arm and bites Kramer on the wrist. As Kramer later was describing the incident to Jerry and George, he was pointing to a bite mark near his elbow.
Factual error: At the end of the episode in which George and Susan get engaged, they are lying in bed together watching "Mad About You" on TV. However, at some earlier point on "Mad About You" it had been established in a crossover episode that Paul Reiser was the person who lived in Jerry's apartment before him and was an old acquaintence of Kramer's, meaning that "Seinfeld" and "Mad About You" existed in the same reality. On "Seinfeld," the show "Mad About You" could not exist.
Continuity mistake: When George and Susan are arguing over dinner about her cousin stealing the name Seven, Susan holds her glass up to her mouth in one shot. In the very next shot, the glass is on the table.
Factual error: Newman says that the "Magic Loogie" incident happened on June 14, 1987, when the New York Mets lost to the Philadelphia Phillies due to an error by Mets first baseman Keith Hernandez. However, the Mets played the Pittsburgh Pirates on that date, and the Mets won 7-3.
Continuity mistake: When George is at Susan's cousins' house complaining about them stealing the name Seven, its bright Daylight out. You can tell because the sun is pouring in through the windows. The woman goes into labor and they all get into the car to go to the hospital. At this point it is nighttime. When they finally arrive at the hospital it is broad daylight again.
Continuity mistake: When Elaine and Jerry are confronting Testicoff about paying for the woman's hospital bill, in the view from behind Testicoff he puts his left hand down to scratch his pants. When the shot switches to a closeup of Testicoff, he is waving both arms above his head.
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.
Revealing mistake: When Kramer is getting slammed with his new super-powered shower head at the very end of the episode, it's blatantly obvious that the water is barely touching him - he's standing right next to it.
Audio problem: As the doctor is explaining how he thinks there were other factors at play to stave off the infection, when he says "something perhaps..." his mouth/lips do not move.
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.
The Cartoon - S9-E13
Continuity mistake: After George and Janet leave The Apartment, Jerry gets mad at Kramer for saying Janet looked like him. He shoves Kramer and the bottom half of the freezer door swings open. The next few shots show Jerry and Kramer from the chest up, but when a wide view shot is shown again, the freezer door is closed. It does not appear Jerry shut the door.
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.