Continuity mistake: In the episode with Courtney Cox, when she and Jerry are arguing over a can opener, a bottle of beer keeps changing positions whenever they cut back to Jerry.
Revealing mistake: When Kramer at the start of the episode puts the hose on the kitchen tap, he jerks the hose as it's not long enough to reach The Fire. If you look at the sink when Kramer does this, it comes easily out of its position on the counter top.
Continuity mistake: When they initially pull into the handicapped parking space, George is driving a large yellow Cadillac. When they return to see the car destroyed, it is now a much smaller blue car.
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.
Factual error: The Devils-Rangers game is at Madison Square Garden, but when the Devils score, a siren goes off. That would only occur in the event of a Rangers goal.
Factual error: In this episode, after accepting a free Armani suit from Kenny Bania, Bania insists that Jerry repay him by taking him to a nice restaurant. Jerry takes Bania to Mendy's, a Glatt kosher NYC deli. During the ordering process Bania suggests, "Oh, get the swordfish. Best swordfish in the city. The best, Jerry." Swordfish is considered an unkosher fish by Glatt kosher standards since it does not have scales, one of the essential criteria. It would therefore never be served at Mendy's.
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 Bottle Deposit (1) - S7-E21
Character mistake: When Jerry tells Kramer and Newman that they left their groceries under the hood of his car, he comments, "The AAA guy said I was this close to sucking a muffin down the carburetor." Jerry's car is a Saab 9000, which are fuel injected.
Continuity mistake: While Kramer is explaining about being in the Woody Allen movie, George is in the background looking out the window trying to stop car alarms with his keys. Less then five seconds later, George ends up by Jerry's counter (without being seen) and has a can of Shasta in his hand. (00:08:40)
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)
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.
Visible crew/equipment: In the final scene with Larry David guiding Noreen off the bridge, a crew member's arm is visible in the bottom right corner of the screen. (00:22:30)
Deliberate mistake: When George is watching Home Alone, the sound effects and music that are heard aren't actually in the film.
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: When George shoos away everyone from The Apartment to make the phone call, in the background you can see the pink bottle with its black lid. When he actually approaches the phone, the bottle is uncapped. (00:09:35)
The Jacket - S2-E3
Continuity mistake: Jerry asks George in front of the fridge "You don't know Robert Schumann?"; between shots his shirt's collar is suddenly fixed, as earlier it was leaving the tie hanging out at the back. (00:05:50)
Continuity mistake: Kramer's moussed up hair looks much different in the first shot compared to the rest of the scene; it's way slicker and frizzles up right after the first cut. (00:00:55)
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.