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 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: 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 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)
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.