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.
Revealing mistake: At the very end of this film, Doc appears in his locomotive time machine. In every shot the plate which says 'ELB' changes color. From red to orange. You would say this is a change in light angle, but when the locomotive rises and turns, you can see every angle of this plate and in every angle this plate is red. (01:48:55)
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. (01:40:10)
Continuity mistake: When Marty is inside the cave (where he hides the DeLorean), keep a close eye on the huge landscape and very large mountains behind Marty when he is screaming at the bear. The entire background changes when he runs out of the cave to just a few hundred yards of ground before it drops down, where Marty falls into the fence. What happened to the vast landscape and mountains? (00:20:30)
Factual error: Each time one of the special Presto-logs begins to burn the locomotive suddenly accelerates. In real life the logs would heat the water faster and generate more steam, but there would be an appreciable time-lag between the time the logs begin to burn and the engine accelerates. Also, train engines must accelerate slowly. If too much power is applied the drive wheels merely spin on the rails. (01:35:20)
Factual error: The steam locomotive in the film is of a 4-6-0 wheel arrangement and is lettered as Central Pacific no. 131. The Central Pacific did have 4-6-0 engines in 1885, but according to the company's locomotive roster, the number 131 belonged to an engine of a 4-4-0 wheel arrangement. Also when the locomotive is uncoupled it shows a Janney (fist style coupler) - that style wasn't invented until 1890. The locomotive would have used a pin and link style connection.