Continuity mistake: When "Mulva" is giving Jerry a neck massage, check out the bathroom door in the background. It keeps changing from open to closed.
Audio problem: As the doctor is explaining how he thinks there were other factors at play to stave off the infection, when he says "something perhaps..." his mouth/lips do not move.
Deliberate mistake: When George is watching Home Alone, the sound effects and music that are heard aren't actually in the film.
Continuity mistake: At the beginning of the episode, the bag on the counter keeps moving and disappearing, and reappearing.
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