Continuity mistake: When Jerry finds The Soup Nazi's recipes in the armoire he hands Elaine at most 4 pieces of paper, but when the shot changes to Elaine the number of recipes in her hands has increased dramatically.
The Pool Guy - S7-E8
Audio problem: When Elaine is arguing with George, she says the sentence "I like her," and the end of the word "her" is audibly cut off while she is still saying it. (00:11:55)
Audio problem: The end of the word "bluey" that George is saying is cut off suddenly at a change to the shot of Jerry. (00:20:20)
The Sponge - S7-E9
Factual error: During the AIDS walk there is an officer seen several times with a patch "Transit Police, NYC." Not only is the patch completely fictional, it doesn't look anything like the real transit police logo. The Transit Police had already been defunct by the time of this episode.
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.