The Robbery - S1-E2
Continuity mistake: When Jerry tells Elaine to "Do it in the shower", he gestures with his fancy retractable umbrella. But he is holding it by opposite ends in the two consecutive shots. (00:02:30)
Continuity mistake: In Jerry's apartment at the beginning, Kramer hands Jerry the telephone. Jerry takes it with his right hand, but in the wide angle he is holding it in the left hand. (00:03:40)
Factual error: Near the end of the show, George is getting in a taxi in Central Park. The taxi medallion is on the side rear roof support. Taxi medallions in NYC are required to be on the front hood.
Continuity mistake: When the man from the hand modeling agency is talking to George about Ray, he lifts up his right arm to make a claw with his hand, and as he does so he has his left arm holding up his right. After a cut, the man's left arm is suddenly hanging down, no longer supporting the other arm. (00:16:05)
The Pool Guy - S7-E8
Audio problem: When Elaine is arguing with George, she says the sentence "I like her," and the end of the word "her" is audibly cut off while she is still saying it. (00:11:55)
Factual error: The grocery store that Kramer walks into with Little Jerry has a round doorknob. Those were phased out of public use in 1992 after the Americans with Disabilities Act took effect, but this episode takes place in 1997. (00:09:45)
Audio problem: When Kramer is telling Jerry about Little Jerry as he exits The Apartment, Kramer finishes his sentence with the word "jealous", and the S is audibly cut off in the middle, as if the audio was spliced there. (00:14:25)
The Fatigues - S8-E6
Continuity mistake: When Frank breaks open an egg in the close-up of the pan, we see there are more than half a dozen eggs in the carton. However, in a wideshot of the kitchen, the carton appears to have only about three eggs. (00:16:15)
Continuity mistake: When George and Jerry are discussing Elaine's poor dancing, The Keys that are sitting on Jerry's kitchen counter inexplicably disappear between shots. (00:04:10)
Continuity mistake: In the theater, before The Movie starts, Brodie has his right hand on the outside of the candy bag in one shot, and suddenly in the next shot his hand is reached into the interior of the bag. (00:07:20)
Continuity mistake: When George is at the window making a complaint, a cut reveals several inconsistencies. The man at the window has his arms in a higher position in the second shot (plus his facial expression is changed), the woman no longer has her hand up on the wall, and George's left hand is suddenly resting on the table. (00:15:55)
The Slicer - S9-E7
Continuity mistake: The slice of meat Elaine is holding up is not swinging in one shot, but after a cut it is suddenly swinging, and after another cut it is swinging more. (00:07:30)
Continuity mistake: Kramer is holding his hands in front of him with a back scratcher in his left hand, and then suddenly after a cut his hands are not up and we don't see him holding a back scratcher. He proceeds to close the door with his left hand, which is now holding nothing. (00:05:25)
Audio problem: When George says "and it will be easy because the receipt is in my good friend," the S in the word "because" is partially cut off at the end, indicating two different recordings were edited together. (00:18:35)
Continuity mistake: When George is about to walk away from Jerry, he begins to close up his wallet, but in the next shot, it is fully open again. (00:18:40)
The Apology - S9-E9
Audio problem: When Jerry is saying to Kramer "that was an hour ago", the beginning of the word "that" is cut off, and it sounds more like "at." (00:02:35)
The Apology - S9-E9
Audio problem: As Kramer is saying "come on, Jerry, how about a baggy swimsuit", the audio does not line up with the timing of his lips moving. (00:06:30)
Continuity mistake: One shot shows George beginning to step out of the car, with his leg out of the vehicle, but in the next shot, he appears to be fully inside the car. (00:06:30)
Audio problem: When it cuts from Elaine to Kramer, the audio of what Kramer says is audibly split, as it sounds like he was speaking previously but it was cut off. (00:11:30)
Continuity mistake: While George is telling the whale story in the coffee shop, the ketchup bottle on the table disappears and reappears between shots. (00:19:00)
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.