Continuity mistake: Kramer just gave Elaine the bench she wanted. The Apartment door is open. Cut to Jerry who says all salty "Well, goody for you!", and the door is closed. It is open again next time it is in frame. (00:17:40)
Continuity mistake: At the beginning of the episode Jerry tells Elaine that he vaguely remembers "doing something"; Julia Louis-Dreyfus puts the remote down and brings her hand up twice in two separate shots. (00:01:30)
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