Revealing mistake: While Jack is leaving The Apartment after giving Jerry The Pen, you can see they have put a fake background outside the front door. What's more it is not even that realistic. It looks like a forest with a river in the middle. The Seinfeld's Florida apartment is situated amongst many other apartments around it.
Continuity mistake: There are inconsistencies regarding how old Jerry and George are. In Season 2's The Heart Attack, George states he is 33 years old. In Season 3's The Stranded - just one year later - Jerry states that he is 36 years old. However, in Season 9's The Butter Shave, it is revealed that they are just four months apart.
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.
Deliberate mistake: Jerry's apartment is on the fifth floor of his building, yet any time he buzzes somebody in from outside the building, they enter his apartment in a matter of seconds.
Continuity mistake: Jerry says he has never seen an episode of I Love Lucy in his life, then pops the cork on the wine bottle. Then it cuts to the lady and she asks, "Is there anything else about you I should know?" and the wine bottle is still on the table, corked. Then it cuts back to Jerry, and the bottle is still in his hand uncorked. (00:11:40)
Continuity mistake: When Mike accuses Jerry of murder at Susie's wake, watch the people behind and around Jerry. There are women either side of Peggy, turning around, looking surprised and moving their hands. The next shot, we see Jerry from behind his seat, and everyone has instantly changed to sitting perfectly still, looking ahead with their hands still.
The Jacket - S2-E3
Continuity mistake: When Elaine's dad says "Pipe down, chorus boy" to George, suddenly a hotel staffer in red appears in the recess by the lamp. In another blatant continuity mistake, in the shot that follows George is suddenly without glasses. (00:17:50)
Other mistake: When Jerry, George and Elaine go out to Long Island to the party, George sees his lady co worker friend who says "guess what property I just sold...129 West 81st". That is Jerry's own address, which wasn't sold to anyone.
Revealing mistake: When Kramer and his lawyer, Jackie, are in the taxi; when Jackie says "who told you to have a pow wow", a man is seen walking across the rear window. The taxi is supposed to be moving, but the man's position stays the same (the taxi should be pulling away from him). The actual traffic filmed for the green screen doesn't seem to react to a man jaywalking in front of them either, as it was someone walking across the set.
The Gymnast - S6-E6
Continuity mistake: When George is at his girlfriend's mother's house, the plates on the table switch around. Then the order he stacks them changes. When he picks them up, the plate with the chicken is in the middle with a plate with a napkin on top, then in the kitchen, the plate with the chicken is on top.
The Calzone - S7-E20
Visible crew/equipment: In the scene where Elaine questions her boyfriend in front of her apartment, look carefully at the end of the conversation. When her boyfriend turns and walks away, you can see where the green screen ends, with the crew members/cameras standing on the set. (00:09:25)
The Slicer - S9-E7
Continuity mistake: At the end of the episode, after George has successfully airbrushed himself out of Kruger's picture and reinserted an authentic picture of Kruger, Kruger picks up the picture and explains to George why he doesn't think the mole Kramer discovered in Kruger's cancer screening is anything to worry about. After George says "Whatever" and just before Kruger begins explaining the story behind the picture to George, he tilts the picture towards the camera and if you look closely you can see that George is still in the picture. (00:11:00 - 00:22:20)
Continuity mistake: Jerry and George are at the pharmacy. As George pays the bill and is heading out, behind Jerry there's an old lady at the counter. The camera changes and the same lady disappears and you can see her in other side of the store heading again to the counter. (00:02:25)
Continuity mistake: In the first scene (approximately 3 minutes in) where Jerry points out the red-headed woman in The Parking Garage to George, she is leaving her vehicle in a section of the garage that is labeled 14 with a green background. However, later (approximately 23 minutes in) she is shown returning to her car in section 14, but the color is now yellow.
Continuity mistake: The inside and the outside of the Costanzas' home do not match up. From the inside, you can see that if you walk in the front door the kitchen would be on your left and a sitting room-type area is on your right (accessible from the living room). However, from the outside, it shows bay windows on what would be the left (kitchen) side and a door to another townhouse on the right (sitting room) side.
The Statue - S2-E6
Continuity mistake: Near the end of the episode, Kramer acts like a cop to get back The Statue. The door is almost completely closed. Just before he leaves, the door is suddenly open about one foot, even though it hasn't been touched yet.
The Strike - S9-E10
Visible crew/equipment: When Jerry is chasing after Gwen and when he returns to the table, you can see the shadow of the boom mic on the wall. It is most noticeable to the right of the framed picture. (00:20:57)
Continuity mistake: At the beginning of the episode, the bag on the counter keeps moving and disappearing, and reappearing.
Continuity mistake: Donna's purse strap changes position on The Couch throughout the scene, when she's there when George brings up her liking the commercial.
Character mistake: George makes a big deal about wanting to say "I love you" to the girl he's dating and says he's never said it to a girl, just a dog once, and he wants to say it just once. But in s02e01, "The Ex-Girlfriend", George tells Jerry he had to say "I love you" to Marlene when she told him she loves him.
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