Continuity mistake: When the DeLorean is being pulled by horses, the clouds in the sky change in the next shot. (00:38:50)
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. (00:16:35)
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)
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)
Revealing mistake: In the overhead shot of the train explosion, you can tell which explosion is 'real' and which one is fake - the second explosion appears out of nowhere from the first one. (01:42:00)
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: Standard gauge for railroad tracks in 1885 was 4ft 8 1/2 inches. The distance between the center of the wheels on a Delorean was 5ft 3 inches. The car would not fit on to the tracks, and we can see the wheels aren't recessed or otherwise altered.
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)