Revealing mistake: While Jack is leaving the apartment after giving Jerry the pen, you can see they have put a fake background outside the front door. What's more it is not even that realistic. It looks like a forest with a river in the middle. The Seinfeld's Florida apartment is situated amongst many other apartments around it.
Revealing mistake: When Kramer and his lawyer, Jackie, are in the taxi; when Jackie says "who told you to have a pow wow", a man is seen walking across the rear window. The taxi is supposed to be moving, but the man's position stays the same (the taxi should be pulling away from him). The actual traffic filmed for the green screen doesn't seem to react to a man jaywalking in front of them either, as it was someone walking across the set.
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.