Continuity mistake: The first time we see Jerry's dad, he's passively taking his wife's whine about The Jacket, remote control on his chest and hands crossed on his belly. Cut, and he is sitting with his arm by his sides. (00:01:00)
Continuity mistake: Kramer leaves with the cantaloupe saying "Well, I am taking this back"; not only he grabbed the fruit during a cut at breakneck speed, but the towel on the station changed also position. (00:17:00)
Continuity mistake: Kramer and Jerry are talking about Marlene. Kramer says "Why would George want to deprive you of pleasure? Is it just me?"; he raised both hands chest-high. At the cut, his right hand is lowered. (00:16:00)
Continuity mistake: In the very first shot of Jerry's conversation with George's ex, behind the pile of books you can spot a big lemon slice on the rim of the glass. The slice is then gone from that spot (it's inside the glass now, submerged by the drink), but between shots it is back on and then off again. (00:08:00)
Continuity mistake: Up in Vermont, Jerry is beginning to have a rather tense conversation with Vanessa. In the close-up after the watch comment (when he is thinking of the delightful drive home ahead) Seinfeld is all smiles, but in the shot before and after he is staring with a tense expression. (00:17:20)
Continuity mistake: Elaine comes out of the bathroom sniffling and with a big stripe of toilet paper. She rubs her left side of the nose when she asks "Didn't work, huh?" In the next shot with George answering with a burst of nervous laughter, Elaine's hand is in a different position, and George himself is clutching a tissue to his chest. (00:16:25)
Continuity mistake: Kramer runs off to meet the girl he just spotted through the binoculars, and George walks through the door and crashes on The Couch. The pillow is straight-ish, very angled and straight again during the conversation, with George's hand changing position between shots. (00:15:45)
Continuity mistake: The scene with Elaine on The Couch plotting the demise of the cats begins with a nasal spray on the kitchen counter. There's no trace of it after that initial shot, and it reappears only when Elaine comes closer for her cup of coffee. (00:13:00)
Continuity mistake: Jerry orders a tuna sandwich, and Elaine turns towards him and whines "Tuna?" Her hand is still on the menu, but when the angle changes her hand is on the neckline of her top. (00:04:50)
Continuity mistake: Jerry says that he wants to suggest to his current girlfriend, Vanessa, to go away for a couple days. George starts saying "No no no" and before the cut he raises his arm and lowers his head as to express concern. But in the new shot there's no trace of that change in posture. (00:02:45)
Continuity mistake: Elaine joins the other late at the table. She sighs and puts her sunglasses on the table, above an open menu. Brief cut to George, and the menu is on top of The Glasses. It is again at the bottom later. (00:01:45)
Continuity mistake: During the initial conversation, George keeps playing with the paper sleeve of a straw in certain shots, while his hand is in another position in the reverse shots (noticeable for instance when he says "Guys with cats...I don't know..." (00:02:30)
The Jacket - S2-E3
Continuity mistake: When everyone gets up from the lobby seats talking to Alton, George's coat is folded on his arm in two different ways that alternate between shots. (00:17:40)
Continuity mistake: Jerry was forced to take the phone call thanks to Kramer. He is holding the receiver in a different way between shots - look at the position of the index, on the microphone in the second shot. (00:03:50)
Continuity mistake: In the hallway scene at the beginning, Jerry is holding a magazine. When the man comes out of the elevator, he is holding the magazine The Opposite way - easy to tell because of the Marlboro ad in the back, now flipped. (00:01:55)
The Robbery - S1-E2
Continuity mistake: Jerry is being grumpy on The Couch with his friends for giving up The Apartment, arms crossed. Notice his right sleeve, down till the wrist. When Carol enters the room with her husband for the first time, Jerry is still with his arms crossed but the sleeve is up, mid-length. (00:20:35)
The Statue - S2-E6
Continuity mistake: When Kramer gathers the old clothes, shoes, hat from the cardboard box on Jerry's table, they keep changing position in his arms between shots. (00:04:30)
Continuity mistake: When Elaine passes the newspaper to George, Jerry is sitting normally in the first shot, leaning with his right arm on the backseat in the next. (00:03:10)
Character mistake: Jerry is shopping with Vanessa; in the store there's a sign with a special offer for the salad bar, "À la cart", instead of "À la carte" (00:05:30)
Continuity mistake: Elaine tells Jerry about the ex with the unclean bathroom. At the end she puts her lollipop in her mouth twice in two separate shots. (00:07:00)
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