Doc: And in the future, we don't need horses. We have motorized carriages called automobiles.
Saloon Old Timer: If everybody's got one of these auto-whatsits, does anybody walk or run anymore?
Doc: Of course we run. But for recreation. For fun.
Saloon Old Timer: Run for fun? What the hell kind of fun is that?
Mary Steenburgen had such a good time dancing with Christopher Lloyd for the festival scenes that she overdid it one day and tore some ligaments in her leg. However, (again) because she enjoyed the dancing so much, she chose simply to fight through the pain. See more...
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Back to the Future Part III (1990) - 55 mistakes
Continuity: When Marty goes back from 1955 to 1885 and meets the Indians, note that none of the Indians shoots any arrows nor do any arrow hit the car. You can see that when he drives the car back into the cave. But when he gets back to the cave after the Cavalry ride, a long arrow sticks out on the side of the car.
Factual error: At the end of the train scene just before Marty travels to 1985, the train busts through a road block warning that there's only 1/4 mile of track left before the ravine. Keep in mind that the train is travelling at approximately 70-80 mph by this time. At 70 mph (and not even accelerating) it would only take the train 13 seconds to reach the ravine. The actual time in the movie is at least a minute.
Deliberate "mistake": When Marty is being chased by the Indians, a wide shot with the DeLorean at the bottom of the screen coming towards us and the Indians following reveals the entire background for a mile or two. However, there are no cavalry visible anywhere despite the fact that they pass over the cave in the following shot only five seconds after the last Indian does.
Factual error: When the red Presto-log is burning, rivets begin to blow out of the locomotive, supposedly from the pressure. The rivets in the boiler, which would be subjected to pressure, are not visible on the outside of a locomotive. The rivets which are seen to blow out in the movie are in the smokebox, an area which is subjected to heat and smoke, but no steam pressure.