The Suicide - S3-E15
Revealing mistake: In this episode when Jerry leaves his room to talk to his neighbour about their fight, when he shuts his door the wall shakes revealing that the place he lives in is not a real apartment.
The Contest - S4-E11
Revealing mistake: In the scene where Jerry and Marla are making out on his couch, at the top right window you briefly notice someone walking by. This is impossible because The Apartment that Jerry lives in is on the fifth floor.
Revealing mistake: When George puts down the hairdryer next to the picture of his "fiance", if you look at the bottom right of the screen, you will notice a blue flame most likely from a torch the crew used to create The Fire.
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.
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.
Revealing mistake: When Kramer is getting slammed with his new super-powered shower head at the very end of the episode, it's blatantly obvious that the water is barely touching him - he's standing right next to it.
Revealing mistake: When Kramer at the start of the episode puts the hose on the kitchen tap, he jerks the hose as it's not long enough to reach The Fire. If you look at the sink when Kramer does this, it comes easily out of its position on the counter top.
Revealing mistake: In the sequence when he gets shot by the FBI agents for stealing cable, Jerry Seinfeld is wearing ear buds, quite visible when he collapses by the door. Evidently the stage props even loaded with blanks were too loud for his taste. (00:09:00)
Revealing mistake: After the scene in which Elaine tells Jerry about her confrontation with the "head-nodding" man, and before the scene where George swallows a fly, the scene cuts to a shot of the exterior of Monk's café. The actual, real-life restaurant used for exterior purposes is named "Tom's Restaurant," and the "m's" in "Tom's" can be seen during this five-second shot. (00:23:00)
Revealing mistake: While Jack is leaving The Apartment after giving Jerry The Pen, you can see they have put a fake background outside the front door. What's more it is not even that realistic. It looks like a forest with a river in the middle. The Seinfeld's Florida apartment is situated amongst many other apartments around it.
Revealing mistake: When Kramer and his lawyer, Jackie, are in the taxi; when Jackie says "who told you to have a pow wow", a man is seen walking across the rear window. The taxi is supposed to be moving, but the man's position stays the same (the taxi should be pulling away from him). The actual traffic filmed for the green screen doesn't seem to react to a man jaywalking in front of them either, as it was someone walking across the set.
The Nose Job - S3-E9
Revealing mistake: When George is talking to his girlfriend about the possibility of getting a nose job, Elaine is visible in the other room waiting to make her entrance.
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)
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)
Revealing mistake: When the two paramedics get off the van to duke it out, look at George lying down in the stretcher. The pillow behind him bears in the last two shots faint purple coloring marks washed out from Jaso Alexander's makeup. (00:18:30)
Revealing mistake: During the shopping scene with Vanessa, the clock on the wall can be seen for over a minute but the hands never move. (00:05:30 - 00:06:40)
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)
Answer: The name is Delores. He shouts that name at the end of the episode. Then in a later episode, "The Foundation", he runs into her. And he repeatedly calls her Delores.