Continuity mistake: When Marty returns to 1985, he goes to his house and gets in his Toyota pick-up. Look closely behind his head when he first gets in the truck, there is no driver-side head rest. But when he picks up Jennifer, when they pull up to the stop sign the head rest is there.
Continuity mistake: Watch the scene with the Indians (just as Marty sticks the DeLorean into reverse) and see the differences of how close and far the Indians are from the DeLorean.
Continuity mistake: 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.
Revealing mistake: At the end, when the train destroys the time machine, right at the moment when it hits you can see a small explosion, which was used to trigger the destruction of the time machine.
Continuity mistake: When the train is coming and is about to run over the DeLorean, the shadow of the train on the wall shows that it is only the locomotive. When the camera changes view, it is a train with freight carriages.
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.
Continuity mistake: When Doc drinks the 'wake up juice', he runs out and puts his head in the horse trough full of water. However, after a shot to Mad Dog Tannen, his shirt is mysteriously dry again.
Continuity mistake: When Marty is about to go back to 1885, he starts driving and you see him pass the building: then when it shows the car's tires, you can see he passes by the same building again.
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.
Continuity mistake: In the last fifteen minutes of the film Marty wears his Wild West clothes, including his brown woolen poncho. The front of the poncho is unmarked, completely intact. Shouldn't it have a bullet hole in it?
Continuity mistake: When Marty is shot by Buford, he is thrown off his feet and lands on his back. As he does, his poncho flips up, showing his shirt all the up to the middle of his chest. The stove door he is supposedly using as a bulletproof chestplate is conspicuously missing.
Continuity mistake: When Clara first sees the train and starts after it on horseback, it is on her right side. She is at the top of a hill and starts down after the train. In the next shot shown from the front of the train you see the horse appear so the train is on Clara's left.
Other mistake: When Marty returns to 1985 from 1885 the time machine is destroyed by a train. What is strange is that not only does the train not stop (which is mandatory in the event of an accident) but there is also no reaction from people nearby, such as those in the vehicles waiting to cross the track at the level crossing. Later, when Marty returns to the site with Jennifer, there is still no sign of police officers or other officials.
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.
Continuity mistake: When Doc's about to send Marty back to 1885 from 1955, and Marty's dressed up...when Doc's walking back and forth loading up the car - look at the shadow from the building. You see it changes by several feet in a few seconds as the scene was obviously shot throughout the day.
Continuity mistake: When Doc and Marty first see each other in 1885, just before they hug, Doc puts his hand on Marty's shoulder, in the next shot it's on his arm.
Continuity mistake: When the DeLorean is being pulled by horses, the clouds in the sky change in the next shot.
Continuity mistake: When Mad Dog is hanging Marty outside the new courthouse, Doc takes aim stood in bright sunlight (when the camera is behind him), but immediately after shooting him down we see Doc from the front, stood in a large shadow.
Factual error: In 1885, Hill Valley and its surrounding areas are shown as having a desert climate, when in both 1955 and 1985, the climate is grassland. In reality, Nevada County, California (where Hill Valley is supposed to be located) is and always has been grassland and forests, not arid desert.
Revealing mistake: When Marty and Shamus are sharing a meal at the dinner table, Shamus passes Marty the plate of rabbit. As Shamus hand moves away to let go of the plate, to the bottom right hand corner of the plate, a purple patch appears briefly where the two M. J. Fox actors interact, an error with the Vistaglide camera tech used back then. Easier to spot on Blu Ray Version, better than DVD.
Continuity mistake: Doc comes back from the past. Jennifer walks to stands next to him. In the following shot, she is still approaching Doc to give him the paper from 2015.
Visible crew/equipment: After Marty runs out of the saloon in 1885 into the street he gets dragged by Buford. As he is being dragged you can see a film crew running beside him. If you have the box set you can kind of see shorts, legs and sneakers, but if you have an older version or see the movie on TV you can see 2 people and the camera.
Continuity mistake: In 1885 Hill Valley, the hills are shown to be almost directly behind the courthouse, however in the previous films there was some distance between the two. This was likely due to the different shooting locations.
Continuity mistake: When the Doc enters the destination time into the time circuits when he is sending Marty to 1885 from 1955, the time/date etc. Go in as he's typing it. However, in the previous films and later in the 3rd film when traveling back to 1985, the enter button must be pressed before anything appears for the destination time.