Revealing mistake: After the scene in which Elaine tells Jerry about her confrontation with the "head-nodding" man, and before the scene where George swallows a fly, the scene cuts to a shot of the exterior of Monk's café. The actual, real-life restaurant used for exterior purposes is named "Tom's Restaurant," and the "m's" in "Tom's" can be seen during this five-second shot. (00:23:00)
Revealing mistake: When the two paramedics get off the van to duke it out, look at George lying down in the stretcher. The pillow behind him bears in the last two shots faint purple coloring marks washed out from Jaso Alexander's makeup. (00:18:30)
Revealing mistake: In the sequence when he gets shot by the FBI agents for stealing cable, Jerry Seinfeld is wearing ear buds, quite visible when he collapses by the door. Evidently the stage props even loaded with blanks were too loud for his taste. (00:09:00)
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.