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.
Other mistake: When Clara is waiting for the train to stop at the train station and the train has steam coming from in front of the wheels, as it passes by the camera it jolts the camera. (01:20:20)
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
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