When Marty is standing over his father's bed in 1955 persuading him to date Lorraine, there is a hair-dryer in Marty's belt that seems to disappear between different frames (note the hair dryer's cord leading to Marty's right hand, as he holds it behind his back). Really obvious. [This scene was originally longer, including a part where Marty removed the hair dryer from his belt. Still a mistake, but there's the explanation.]
When Marty is being chased by the Libyans, a few times it cuts away to show the instrumentation in the car. The first time the odometer reads 33064.2 and the trip counter says 88.8; the second time it's goes back a little to 33061.8 and 86.4. A few seconds later we see it as he turns the corner, and it goes down again from 33,061.8 to 32,994.4, the trip counter has gone from 86.4 to 19. In fact it's a completely different set of instrumentation - the bottom of the needle's angled in some shots and flat in others.
When Marty is in 1955, he is in shock and walking on the pavement when a car comes by. On the first shot when it comes towards him it has two speakers, on the second shot when it's moving away, it only has one. On the second shot of the car with only one speaker, the A-frame sign is also turned the other way.
When Marty's coming home at the beginning of the film, he's watching the discussion between his father and Biff who complains about the wrecked car and his reports. In the two shots which show Marty, there's a glass full of candies near to him. When Biff steps to the glass and lifts the lid, half of the candies are gone.
When Lorraine gets George's book out of the package, the bracelet she is wearing on her right arm is down by her wrist in the close-up. In the next shot as George takes the book from her, that bracelet has moved up her arm and is hidden under her sleeve, so when she puts her arm down and then places her arm around George's waist we see that the bracelet has slipped back down to her wrist. (It doesn't disappear, it changes positions between shots.)
When Marty's been thrown back by the amp, the papers on top of him change between the first and second shot. Precisely a Polaroid photo, a poster/map and a vase are missing.
At the end, the DeLorean hits the left dustbin and comes to a halt - you can easily see that it's not Christopher Lloyd driving, it's someone with a much bigger build, wearing an awful white wig.
As the Delorean accelerates towards Doc and Marty in the car park, in a shot from the car's perspective they're a few feet apart, then we cut to a close up, and they're right next to each other.
At the beginning of the movie after Biff leaves the McFly home. There is a scene where Marty and George are talking. In one shot George has three pens in his pocket, a few seconds later he has two.
In the cafe, the menu in front of Marty changes after Biff leaves. It starts looking like an ice-cream menu, with swirly lines at the top, but just before the future mayor comes into shot, the design on the front changes completely.
See all the mistakes in Back to the Future
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