Continuity mistake: In the scene where Moe shows the Tenor brothers their apartment, the star on the door is gold. Later when Morty is riding around on the scooter, you can see the star on their door has changed to silver. It changes many more times throughout the movie.
Continuity mistake: Whilst Walt and Bob are cooking at the beginning of the film, there is a large bandage on Bob's forehead. Just after chopping up the mushrooms the bandage disappears, the next time we see them it is back on. (00:03:05)
Continuity mistake: Right before Bob has his first panic attack, Walt is holding the newspaper in his left hand. When the shots change, it ends up in his right hand without him moving it.
Revealing mistake: At the beginning when cutting the tomatoes onto the buns, Bob is not even coming close to the tomato in his hand, yet slices are falling off onto the buns. (00:03:50)
Continuity mistake: In the diner, when the camera shoots the twins from the other side of the table, a sugar bottle can be seen. However, when it shoots from Bob's side of the table a bottle of hot sauce can be seen (it should be seen from the other angle as well, but is absent). (01:16:50)
Continuity mistake: When Bob has his first panic attack (about Walt's one-man play), he takes a glove off to hold the paper bag, but a few shots after he's wearing both gloves, without putting the glove back on.
Continuity mistake: At the beginning the ketchup and mustard dollops put on the buns originally are not the same as when they start flipping burgers onto the buns a bit later. (00:03:50)
Continuity mistake: When they are getting on the bus after being turned down by some agents there is a guy with a red shirt getting on the bus. The scene cuts and shows the guy with the red shirt smoking but not on the bus. The other people in the shots are mixed up, too (the only one that stays the same is the girl Walt hits on). (00:25:15)
Continuity mistake: When Morty first appears, almost all his below teeth are gone, however throughout the movie this keeps changing, from having all from having one to none.
Factual error: When May finds out that Bob and Walter are conjoined twins, they are holding a Dunkin' Donuts bag. There are no Dunkin' Donuts in Los Angeles.
Continuity mistake: When Walt is telling Bob that he wants to be an actor, the tassel on Bob's hat changes from angling back to looping forward. (00:15:55)
Revealing mistake: In the intro of the series "Honey and the Beaze" which Walt participates in, you can see Walt standing by a wall that reflects him, but there is no sign of Bob that usually would stand beside him. (01:01:10)
Revealing mistake: In the beginning when squirting ketchup and mustard on the buns, the angles the bottles are held at and how the condiments come out of them show that he is not actually squirting anything. It's all done with computer graphics. (00:03:50)
Revealing mistake: Right after May finds out Bob and Walt are conjoined she peels out in her car. In a close up on the twins you can see on Walt's right shoulder the suit they glued to him to conjoin the two. (01:09:05)
Continuity mistake: At the end when Bob walks out to see who played the song, there is an order pad sitting near the end of the counter. When Bob walks out with Walt the pad has turned into what looks like a menu and moved to the very corner. (01:42:55)
Continuity mistake: When May is talking to Bob in private her hair goes from being over both shoulders to being over one, to being over both, etc. (01:24:15)
Continuity mistake: In the shot where Bob and Walt are in jail and Walt is eating, you can see a cop holding a cup of coffee with both hands. In the next shot he is holding it with only one hand. (01:17:05)
Continuity mistake: When the boys are first laying out the buns the second bun from the left in the front row has a yellow stain on it. When it comes back to them about to put ketchup and mustard on the buns the stain is missing. (00:03:25)
Continuity mistake: While May and Bob are talking in private, there is a clump of leaves in the pool that appears near the life jacket, which wasn't there in an earlier scene. (01:24:05 - 01:25:55)
Continuity mistake: When Bob and Walt are having lunch you can spot a bottle of Tabasco on the counter beside them. In the next shot the Tabasco is gone. Then it appears again. (01:13:50)
Answer: It's the Billy Stewart version of "Summertime" from George Gershwin's opera "Porgy and Bess".
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