Continuity mistake: When Buford chases Marty, the position of his 3 goons is an absolute mess and changes from shot to shot with no continuity whatsoever: Sometimes they're behind Buford; or two riders on the right and one on the left; or last but not least: all three riding past Marty until a shot later one of them suddenly appears way behind the group.
Continuity mistake: When Buford chases Marty, note the second building after the bar, the Stetson house with a semicircle on the roof. Buford passes by this place at least three times during the chase.
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.
Continuity mistake: When Marty hides from the Native Americans by parking inside a cave, the windshield is covered in dust, but from a brief angle where Marty watches them jumping over the mount, the glass is spotless.
Continuity mistake: Marty encounters the Native Americans and a chief holds a spear horizontally above his head for several shots, but in in the immediate wide angle a frame afterwards, the spear is gone, or a at least no-one is holding one horizontally.
Continuity mistake: When Marty drives in reverse to run away from the Native Americans there's one on the right side ahead of the car. When the angle changes he's disappeared and all of them are behind the car.
Continuity mistake: When Marty is driving toward the cinema screen to go to 1885, after thunderbolts surround the car, in the front shots he looks concentrated and calm, but in a brief shot from the side he is really scared and tense. This swaps back and forth.
Deliberate mistake: When Marty drives towards the screen to go back to 1885, he passes by the same 5 speakers over and over again. It's obvious that the road was very short and they repeated the shots from many different angles to add-up for tension.
Continuity mistake: When Doc gives Marty the picture of them, Marty opens the package and leaves the wrapping paper behind. A shot later, from a a different angle, the paper is all around the picture.
Continuity mistake: When the movie is about to end and Marty and Jennifer walk by the tracks they have a bumper in front. Jennifer says "there's not much left", then the angle changes and the bumper has magically moved behind them.
Continuity mistake: After the breakfast machine dumps the eggs in the pot at the start, the eggshells disappear.
Continuity mistake: When Marty and Doc are jumping on the logs on the train, the sun changes between shots.
Factual error: After Doc and Marty hijack the train, there are shots of them in the cab of the locomotive. Look closely behind them, there is a clip to hold a microphone for a 2-way radio or CB attached to the window post, which shouldn't be there in 1885.
Continuity mistake: While Marty is fighting Tannen the sun changes between shots.
Continuity mistake: In the end scene where Doc's train comes back to 1985, Marty and Jennifer are literally blown away by the re-emergence of the train. While they are flying in mid-air, the front of the train is passing them by and gets just about to the cab (where the ELB is). However in the next cut, Marty and Jennifer are on the ground watching as the train just starts passing them again, and the train comes to rest at a point before the previous "blown away" cut scene.
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