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: George teleports from being on the other side of the room to standing right next to Jerry after a cut. (00:02:45)
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)
Plot hole: When Elaine meets Glenn's welfare case worker at The Apartment, the case worker does not tell Elaine that Glenn actually has a wife (after clarifying that she isn't The Wife). This is pure plot convenience. When Glenn's wife finds Elaine in The Apartment, he says "You're home early." She lives there, they are not separated, and surely the case worker would be involved with both of them, not just Glenn.
Factual error: Elaine is placed in the same jail cell as the others. No police department anywhere in the U.S. would put a woman in the same cell as men.
Revealing mistake: When Kramer is driving his Tupperware full of blood in Jerry's car, you can tell by how rapidly The Blood is oscillating that it is just red-colored water. As the saying goes, blood is thicker than water, so actual blood wouldn't oscillate that rapidly, especially when it had just been pulled from a freezer. (00:18:20)
Factual error: When Peterman is making a toast at his party, he quotes what he says is something said by the Papa New Guineans, and he proceeds to pronounce a sentence filled with clicks (lingual-ingressive consonants). However, this is impossible, as clicks are only used in southern African languages and three east African languages. (00:03:10)
Revealing mistake: When Jerry is slamming the payphone in frustration with Elaine, he slams it on The Switch-hook multiple times. Pressing down The Switch-hook like that would end the call. Clearly Jerry is not really connected to Elaine on the phone. (00:14:05)
Continuity mistake: When George enters The Cafe near the start, he takes Jerry's plate and begins eating the fries. After finishing the first fry, his hands are on the plate. Jerry then asks him what happened, and suddenly he has a fry in his hands which are also now raised slightly.
Factual error: With the size of that red ball (approximately 2.5 foot diameter) and the density of oil (approximately 60 lb/ft^3), the giant ball of oil would weigh 491 pounds. This would be impossible for Kramer and Darren to lift out of the window and would definitely kill anyone it beaned.
The Apology - S9-E9
Continuity mistake: When Jerry and his naked girlfriend are playing scrabble just after Jerry accuses her of stealing and it flips to the front at the bottom of the screen you can see a bra or towel covering her breasts but is gone when she stands up. (00:03:40)
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.