Continuity mistake: When Loretta comes into The Apartment she is carrying a coat and roses in one hand and the other hand is supporting her handbag by her right shoulder (left on screen). In the next shot she is holding the bottom of the flowers with both hands instead of just the left hand.
Continuity mistake: When Kramer and Jerry are talking about the bird being in the hallway, notice how the intercom keeps changing. In some shots the intercom is not set straight and leaning to the right and in other shots it is completely straight. The most notable change is when Kramer is leaving The Apartment. It goes from crooked as he is leaving, then straight while he is outside in the hall and then the entire unit is completely pulled right out after Kramer rushes backs in.
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