when Jerry's father calls and Jerry answers using his portable phone, the antenna is down, when the camera shifts from Jerry to Elaine, then back again, the antenna is up.
During lunch with Leo, when he refers to Lydia as "a real tiger," he sets his drink down. The camera then focuses on Jerry and it can be seen that Leo's drink is suddenly raised to his lips.
In the diner, the first shot of Uncle Leo's burger shows that was bitten. When they flashed back to it , it was whole and unbitten. The next shot shows Uncle Leo biting into an unbitten burger. The scene is out of sync.
When Peterman comes into Elaine's office to tell her she tested positive for opium, he lowers his head to look directly at Elaine while talking to her. In the next shot, his head is raised and he is looking just over her head.
Kramer complains that his shower head has no pressure and he had to take a bath. In actuality, Kramer only takes baths, not showers.
Not one part of Mr. Peterman's reminiscing about his dabbling in opium on a 1979 cruise up the Yangtze river is the least bit credible. Opium use was forcefully eradicated by China's Communist government in the early 1950s. And 1979 was a time of upheaval during the aftermath of the Cultural Revolution and Chairman Mao's death, when tourism was non-existent.
The Masai 'bushmen' (who live less in the bush and more in the Rift Valley) don't wear sandals as a rule, mostly going barefoot. If they do wear them, they are made from tyres.
After Jerry drops his bottle of beer and it smashes, it is obvious that the bottle never contained any liquid. Both it and the floor are completely dry.
In the final scene when Kramer uses his new shower head, it's obvious that he is veered off to the side, as he isn't being hit by the water.
in the scene where Mrs Seinfeld is asked by Elaine to wee in a cup, Mrs Seinfeld goes to the kitchen and opens the glass cupboard, you can see the reflection of the camera crew equipment many times in the glass door.
Elaine says she is off to Kenya to meet the bushmen of the Kalahari. The Kalahari is a desert on the borders of South Africa, Namibia and Botswana, nowhere near Kenya.
Elaine (Julia Louis-Dreyfus) eats some chicken Kramer (Michael Richards) offers her. In episode 3.10, The Stranded (1991), she claimed the only meat she'd eat was fish.