Continuity mistake: When Buford chases Marty, note the second building after the bar, the Stetson house with a semicircle on the roof. Buford passes by this place at least three times during the chase.
Sacha
Continuity mistake: When Buford hangs Marty, after Doc arrives watch Buford's horse's position relative to the wooden crate: it keeps increasing/decreasing randomly between shots.
Sacha Continuity mistake: When Marty hides from the Native Americans by parking inside a cave, the windshield is covered in dust, but from a brief angle where Marty watches them jumping over the mount, the glass is spotless.
Sacha Continuity mistake: Marty encounters the Native Americans and a chief holds a spear horizontally above his head for several shots, but in in the immediate wide angle a frame afterwards, the spear is gone, or a at least no-one is holding one horizontally.
Sacha Continuity mistake: When Marty drives in reverse to run away from the Native Americans there's one on the right side ahead of the car. When the angle changes he's disappeared and all of them are behind the car.
Sacha Continuity mistake: When Marty is driving toward the cinema screen to go to 1885, after thunderbolts surround the car, in the front shots he looks concentrated and calm, but in a brief shot from the side he is really scared and tense. This swaps back and forth.
Sacha Continuity mistake: When Doc gives Marty the picture of them, Marty opens the package and leaves the wrapping paper behind. A shot later, from a a different angle, the paper is all around the picture.
Sacha Continuity mistake: When the movie ends, Marty and Jen are blown away. They're either landing on shadowy or sunny grass, depending on the angle. This changes back and forth.
Sacha Continuity mistake: When the movie is about to end and Marty and Jennifer walk by the tracks they have a bumper in front. Jennifer says "there's not much left", then the angle changes and the bumper has magically moved behind them.
Sacha Continuity mistake: When the movie is about to end, Marty picks up Doc's photo. From the front shot he is grabbing it by the middle, but in the opposite angle he holds it by the lower end.
Sacha Continuity mistake: After the breakfast machine dumps the eggs in the pot at the start, the eggshells disappear.
oswal13Continuity mistake: After Marty gets to 1885, a bunch of Indians are running to him and the cavalry behind them, but in the open view one second before, the cavalry was not there.
oswal13Continuity mistake: In 1955 while the Doc is recording some stuff, while Marty behind him is sleeping on the couch, his head moves from side to side between shots.
oswal13Factual error: When Marty travels back to 1885 and meets the Native Americans, we see a view from inside the car with the digital speedometer. The car is still going over 70 when the Indians are less than 2 metres away from the front of the car. There is no way the car would be able to stop that quickly from that speed.
Continuity mistake: After visiting Seamus, Marty walks to the Hill Valley train station where a man is by the ticket window. In the next shot the man has disappeared.
Sacha Other mistake: After Mad Dog tries to kill Marty at the bar, a man says "you better run, squirrel." Marty is crouched, 4 meters away from the counter, but in half a second he has managed to stand up, run to the counter, jump on it and run towards Buford.
Sacha Deliberate mistake: When Marty drives towards the screen to go back to 1885, he passes by the same 5 speakers over and over again. It's obvious that the road was very short and they repeated the shots from many different angles to add-up for tension.
Sacha Continuity mistake: When Marty and Doc are jumping on the logs on the train, the sun changes between shots.
oswal13Factual error: After Doc and Marty hijack the train, there are shots of them in the cab of the locomotive. Look closely behind them, there is a clip to hold a microphone for a 2-way radio or CB attached to the window post, which shouldn't be there in 1885.
Factual error: When the Indians are running from the cavalry they cross over a set of tire tracks. They are too far from where Marty started so cannot be his, and the lines can't be wagon tracks because the lines are too wide and the width is uniform. (00:19:30)
jimba
Chosen answer: According to the letter, Doc plans to live out his life in 1885. But as it turns out, Doc gets murdered a week after sending the letter. Knowing that Doc's plan to live out his life don't come to fruition, and because he's a good friend, Marty travels to 1885 to save him.
JC Fernandez