The Chinese Restaurant - S2-E11
Continuity mistake: Elaine is talking quite loudly about to go eat at Sky Burger; the payphone is not in use. Jerry replies he does not want to, and suddenly a person is on the phone. (00:19:30)
The Busboy - S2-E12
Continuity mistake: At the end of the episode Elaine, George, and Jerry are sitting in the coffee shop and the ketchup moves from inside by Jerry to the outside of the table when Elaine's to-go order is put on the table.
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.