Factual error: During the entire series, the view of Jerry's door from the hallway is of a flat wall with the door in line, yet from inside the apartment, not only does the kitchen angle into the hallway, but Jerry's fridge is recessed into the wall. The wall behind it should be blocking the hallway completely.
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.
Plot hole: In the episode where Kramer decides to start sleeping like a Seal, he's with a girl. He falls asleep on top of her and she seems to think that he's dead. While breathing, your chest expands in such a way that if somebody were laying on top of you, there is no conceivable way that you could think that they are dead.
Continuity mistake: There are inconsistencies regarding how old Jerry and George are. In Season 2's The Heart Attack, George states he is 33 years old. In Season 3's The Stranded - just one year later - Jerry states that he is 36 years old. However, in Season 9's The Butter Shave, it is revealed that they are just four months apart.
Add timeBrittle Fingers
Continuity mistake: The inside and the outside of the Costanzas' home do not match up. From the inside, you can see that if you walk in the front door the kitchen would be on your left and a sitting room-type area is on your right (accessible from the living room). However, from the outside, it shows bay windows on what would be the left (kitchen) side and a door to another townhouse on the right (sitting room) side.