Revealing mistake: When George puts down the hairdryer next to the picture of his "fiance", if you look at the bottom right of the screen, you will notice a blue flame most likely from a torch the crew used to create The Fire.
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.
Answer: Composer Jonathan Wolff used a synthesizer, although in seasons 7-9, a real bass is used in addition. Wolff also recorded himself making hundreds of mouth noises, pops, and slaps to add to the synthesized bass licks so that each episode has a different theme. The only real "back-story" is Jerry Seinfeld was having trouble coming up with a theme song and talked to a friend who happened to know Wolff. They wanted to avoid that cheesy late 80's sit-com theme song and Wolff came up with what we enjoy now. Jonathan Wolff has also talked about this further in interviews, recently Reed Dunela interviewed him, so for a fuller account of his story; check out "The Wolff of 116th street".
Bishop73