Continuity mistake: The bite size of the cookie that Jerry is eating changes between shots, without him even taking another bite. Not to mention the white and black change sides.
Continuity mistake: When Jerry is eating the black and white cookie in the bakery, sometimes the vanilla side is on the left and sometimes it is on the right. It keeps switching between shots.
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