Continuity mistake: When George is about to walk away from Jerry, he begins to close up his wallet, but in the next shot, it is fully open again. (00:18:40)
Continuity mistake: One shot shows George beginning to step out of the car, with his leg out of the vehicle, but in the next shot, he appears to be fully inside the car. (00:06:30)
Continuity mistake: When George is with his girlfriend at the coffee shop, the ketchup bottle on the table switches between being knocked over and stood up in different shots, despite nobody ever touching it. (00:03:10)
Continuity mistake: During a conversation with Kramer, Jerry is holding a drink in his left hand. Although he is never seen setting it down, the drink disappears in the next shot and he has his arms crossed. (00:02:10)
Continuity mistake: After Kramer walks into Jerry's apartment, he shuts the door. However, in another shot a couple seconds later, the door is inexplicably wide open. (00:01:45)
Continuity mistake: While George is telling the whale story in the coffee shop, the ketchup bottle on the table disappears and reappears between shots. (00:19:00)
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)
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