Continuity mistake: While Jerry is having his massage, The Masseuse walks around the table to reach his right side and we can then see two items of clothing, one blue and one grey, hanging on the clothes stand behind them. During the rest of the scene, we only ever see the grey item hanging there.
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.
Continuity mistake: When Kramer spills the tomato juice all over Jerry's walls, he grabs a light colored oven mitt to wipe it off. In one shot he wrings out the sopping mitt over the juice pitcher. When the shot changes and he tosses the mitt back onto the counter, the mitt is still the original color with no hint of red stains.
The Busboy - S2-E12
Continuity mistake: At the end of the episode Elaine, George, and Jerry are sitting in the coffee shop and the ketchup moves from inside by Jerry to the outside of the table when Elaine's to-go order is put on the table.
Continuity mistake: When George is in the boardroom, he places his briefcase on the conference table, and opens it, with the top leaning against a carafe. As George yammers on, the briefcase is closed in a couple of shots, and then open again for the rest of the scene.
Other mistake: When Mr. Bavalaqua is telling Jerry and Duncan the rules for the big race, there is a male extra right behind him who is mouthing Bavalaqua's lines as he's talking. (00:21:00 - 00:22:00)
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.
Revealing mistake: When Jerry and Kramer are in the theater, Kramer trips and spills his cafe latte all over his shirt. But you can see the coffee stain on his shirt before he trips and spills it.
Continuity mistake: When Elaine crushes the beer can in Jerry's apartment, she leaves it on the counter. After she walks out of The Apartment, the beer can is on the cutting board.
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: The exterior shot they use when establishing Monk's cafe is completely different than what is inside in The Cafe. First the outside has neon lights with just RESTAURANT. The inside of The Cafe, the menus, windows and door all have Monk's cafe logo on them. Secondly the door should be in the middle of the restaurant, instead the interior shows it to be to the left of the restaurant and somewhat to the side. Lastly the windows in the exterior shot are extremely large with no writing, logo and curtains pulled to the side. Inside The Cafe, the windows are much smaller, with permanent curtains coming halfway up the windows and the rest being blocked by Monk's cafe logo.
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.
Continuity mistake: At the end of the episode, Kramer is telling Olive that he met someone else. Olive says "who is she?" and she puts her hand on her hip, in the next shot both of her hands are back on the counter.
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.
Continuity mistake: At The Airport, while deciding to take The Limo, Jerry's scarf moves on and off his shoulder between shots.
Continuity mistake: In the episode with Bette Midler, she is injured when George tackles her at home plate. When he hits her, she is knocked into somebody. When they show her again, she is sprawled against the backstop with no one behind or even near her that seems to be hurt. George also misses home plate.
The Sponge - S7-E9
Audio problem: When Kramer says "I read it in Wall Street Week," his mouth does not sync with the words. It looks like he said "Wall Street Journal."
The Library - S3-E5
Character mistake: When Marion mentions Mr Bookman she says he'd been on the job for 25 years. Yet later on he tells Jerry he's been on the job since 1971, which in 1991 makes it 20 years.
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.