Continuity mistake: After his gig, when Seinfeld is talking to Kramer in the backstage, his T-shirt is all wet around the neck and chest, and a couple of seconds later it appears dry.
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.
Continuity mistake: When the NBC executives introduce themselves to Bania at the comedy club, their last names have swapped since season 4. Jay introduces them as Jay Chermak and Stu Crespi, when in Season 4, they were Jay Crespi and Stu Chermak.
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.
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)
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.
Other mistake: In the scene where Jerry, George, and Elaine are in the coffee shop and Kramer walks in with the dummy mailman with the bucket over his head, Kramer remarks how nobody entered the post office while he was outside protesting. Jerry remarks, "That is because it is Sunday." If that is true, how can the group be gathered at the coffee shop? There is a red sign by the entrance of the coffee shop showing the business hours, and the coffee shop is supposedly closed on Sunday.
Continuity mistake: When Elaine and her new boyfriend are eating in the restaurant, her bangs are to the side. The shot goes to a close-up of her boyfriend, in the next shot her bangs are across her forehead. Then in the next close-up of her face, her bangs are back to the side.
Continuity mistake: When Elaine is sitting with Puddy in the restaurant when they get back together, the amount of wine in Elaine's wine glass keeps changing.
The Slicer - S9-E7
Continuity mistake: When Elaine is woken up by her neighbor's alarm she looks at her own bedside alarm clock and it shows 3.30 AM. In the next shot when she buries her face in her pillow and screams, that clock shows 3.29 AM. (00:00:50)
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)
The Slicer - S9-E7
Continuity mistake: At the end of the episode, after George has successfully airbrushed himself out of Kruger's picture and reinserted an authentic picture of Kruger, Kruger picks up the picture and explains to George why he doesn't think the mole Kramer discovered in Kruger's cancer screening is anything to worry about. After George says "Whatever" and just before Kruger begins explaining the story behind the picture to George, he tilts the picture towards the camera and if you look closely you can see that George is still in the picture. (00:11:00 - 00:22:20)
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
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)
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)
The Strike - S9-E10
Visible crew/equipment: When Jerry is chasing after Gwen and when he returns to the table, you can see the shadow of the boom mic on the wall. It is most noticeable to the right of the framed picture. (00:20:57)
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 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)
Factual error: George asks for a nickel from Jerry for his candy bar. He then proceeds to put only 3 coins into the machine for a $0.50 candy bar. No 2 coins would require a nickel to make 50 cents.
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)
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.