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: After Kramer walks into Jerry's apartment, he shuts the door. However, in another shot a couple seconds later, the door is inexplicably wide open. (00:01:45)
Continuity mistake: During a conversation with Kramer, Jerry is holding a drink in his left hand. Although he is never seen setting it down, the drink disappears in the next shot and he has his arms crossed. (00:02:10)
The Outing - S4-E17
Factual error: Kramer gives Jerry a two-line phone for his birthday, which Jerry plugs in and immediately uses. But a two-line phone requires the installation of an actual additional phone line. What Jerry was using when the NYU reporter overheard him "admitting" he was gay was simply call-waiting.
The Old Man - S4-E18
Revealing mistake: After The Old Man's dentures are destroyed in the garbage disposal, he shouts that his dentist is "downtown." There is sibilance audible in his production of the T sound (after the stop release). Sibilance is high frequency noise cause by turbulent airflow directed at the front teeth vibrating randomly in a small space; if he had no front teeth, there should be no sibilance. (00:14:35)
The Implant - S4-E19
Continuity mistake: During the wake for Betsy's aunt, George puts snacks on his plate while talking to the reverend. He puts at least 6 pieces on his plate while the next shot of his plate shows only 2. (00:14:15)
The Implant - S4-E19
Continuity mistake: After George puts a couple of the snacks in his mouth, he takes a bite of the next one before Father Jessup says George is great comfort. In the next shot, the snack George is whole and no longer has a bite taken out of it.
Continuity mistake: When "Mulva" is giving Jerry a neck massage, check out the bathroom door in the background. It keeps changing from open to closed.
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.
Deliberate mistake: When George is watching Home Alone, the sound effects and music that are heard aren't actually in the film.
Continuity mistake: At the beginning of the episode, the bag on the counter keeps moving and disappearing, and reappearing.
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: When Kramer enters Jerry's apartment, he closes the door. In the next shot, the door is wide open. A few shots later, George opens the door, which was closed when he entered.
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 Glasses - S5-E3
Continuity mistake: When George bites into the onion, you can see the top of the onion is facing up or towards the camera, next to his finger. In the next shot, you can tell the onion has rotated position. You can see the root is to the side, so the top would be on the side instead of up.
Continuity mistake: When Jerry and Elaine are in the kitchen talking about circumcision, in some shots there is cereal on Jerry's spoon, and in some shots there isn't.
Continuity mistake: In the episode where Kramer becomes a ball-man, the episode ends with them at a tennis match. It shows a shot of the court, and no one is standing anywhere near the poles of the net. In the next shot, Kramer is crouched down next to the pole, ready to dive for the ball.
The Cigar Store Indian - S5-E10
Continuity mistake: George has sex in his parent's bed, explaining "My bed is too small." But in another episode, we see George's parents bedroom, and they sleep in separate twin beds, not a big bed.
Continuity mistake: The bite size of the cookie that Jerry is eating changes between shots, without him even taking another bite. Not to mention the white and black change sides.
Continuity mistake: When Jerry is eating the black and white cookie in the bakery, sometimes the vanilla side is on the left and sometimes it is on the right. It keeps switching between shots.
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.