Character mistake: When George and Elaine go to the coffee shop after The Movie without Jerry, they establish that Elaine moved to NYC in 1986. Then they make fun of the way that Jerry throws up; both imitating Jerry. The "Dinner Party" episode is after the "The Dog". In the "Dinner Party" episode, Jerry brags that he hasn't thrown up since June 29th, 1980. The black and white cookie ended this streak. Elaine would not have been around to see this, not arriving in NYC until 1986.
Continuity mistake: Shot shows a DC-10. Next shot is the inside of a plane that is not a DC-10. Only one aisle. DC-10s have two aisles. (00:00:35)
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