Continuity mistake: George teleports from being on the other side of the room to standing right next to Jerry after a cut. (00:02:45)
Continuity mistake: Kramer is holding his hands in front of him with a back scratcher in his left hand, and then suddenly after a cut his hands are not up and we don't see him holding a back scratcher. He proceeds to close the door with his left hand, which is now holding nothing. (00:05:25)
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: When Newman and the landlord look into Kramer's apartment through his peephole, the background doesn't match from when they look in and when he opens the door.
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