Continuity mistake: When Jerry's neighbours are arguing in the corridor about sponge size, Harold is holding the tiny scourer in different ways between shot (in a different spot and with his fingers suddenly stretched out). (00:08:40)
Continuity mistake: At Monk's, Jerry points at the woman feeding corned beef hash to her baby. There's an angle with the camera roughly behind Jerry Seinfeld's shoulder; the continuity here is sloppy, with George having his hand down flat on the table instead of with the fingers laced together with the other, and he has his right leg crossed and not the left. Just few seconds later he asks Jerry if he remembers his friend from Detroit, and he is pointing his left index finger at Jerry across the table instead, again, of having his hands close together. (00:07:30)
Continuity mistake: Kramer's moussed up hair looks much different in the first shot compared to the rest of the scene; it's way slicker and frizzles up right after the first cut. (00:00:55)
Other mistake: Jerry is at the party Elaine's friends have during the marathon. The clock in their living room never changes the displayed hour. (00:15:20 - 00:18:50)
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.