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. (01:43:03)
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. (00:24:10)
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)
Revealing mistake: At the festival, after Marty is given the gun he encounters Seamus and Maggie. Due to the Vistaglide camera effect where Marty was later added to the scene, she is unable to look at where Marty is supposed to be. This is very noticeable when Seamus says "buffoon" and Marty turns around and starts to walk, but Maggie keeps staring at where he was. Also when the scene ends and she looks at Marty and warns him to watch for the future, she's totally missing the spot.
Revealing mistake: When Marty pours the wake-up juice through the funnel into Doc's mouth, in the close up the funnel is straight, but we can't see Doc. From a different angle where we do see the funnel stuck in his mouth, check the right side of the screen: The funnel is skewed (and seemingly it must have a tap) and the liquid is falling out in large amounts to avoid the actor from drinking it.