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: During the sequence with the Native Americans chasing the Delorean in the beginning, in one shot looking up at the sky you can see contrails from a plane, obviously not around in 1885. (00:19:25)
Revealing mistake: At the end when Needles tries to race Marty, Marty goes backwards and does a reverse 180. When they show the whole truck right when it's turning, you can see the tracks from the previous takes and the dust that helped the truck spin. (01:47:30)
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: When Marty is in the cave with the bear, there are two non-consecutive shots from behind the bear. You can see scruffy, fake-looking fur on the bear in those two shots - obviously someone in a black fur costume. The rest of the shots use a real bear.
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 Clara's dress rips off and is about to fall off the train, Doc saves her. One can notice it's not Christopher Lloyd but his stunt double. (01:41:17)
Revealing mistake: When Doc jumps from the horse to the train, right after he hurries Marty, the man on the horse is an obvious stunt double for Michael J. Fox.
Revealing mistake: When the train pushes the DeLorean and hits 70 mph Doc tries to grab Clara. In the wide shots (where we can't see his face) his coat whips, but in the close-ups where we do see his face the coat flutters, revealing that the train was going much faster with the stuntman.
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 actually skewed (and seemingly it must have a tap) and the liquid is falling out in large amounts to prevent the actor from drinking it. (01:20:04)
Revealing mistake: When Clara horseback rides towards the train, when she reaches for the handlebar she is replaced by a very obvious stuntwoman who looks nothing like her.
Answer: He simply has an absurdly low tolerance for alcohol, and whiskey is not a wise choice if this is the case. It helps set up the joke when Marty asks the bartender, "How many has he had?", and he replies by telling Marty, "Just the one", as we are meant to think Doc has been in the bar all night drinking away his sorrows.
Jazetopher