Continuity mistake: When Kramer is telling the fantasy camp story to Jerry and Elaine, the suitcase in the background is open different amounts during different shots. (00:09:55 - 00:10:40)
Audio problem: When it cuts from Elaine to Kramer, the audio of what Kramer says is audibly split, as it sounds like he was speaking previously but it was cut off. (00:11:30)
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.