Continuity mistake: After the modern train destroys the DeLorean, Marty lays on the grass, face down. Then, in a close-up, he turns around to watch the train. When the angle changes he is still facing down and crawling on the grass. In the following shot he has moved 5 meters to the right and is sitting by the bushes close to the tracks.
Continuity mistake: When the modern train is about to destroy the DeLorean, Marty exits the car jumping head first onto the ground. In the immediate next angle he has magically moved 4 meters to the left and is standing tall, jumping over the bushes towards the lawn.
Continuity mistake: When the railroad is about to end, Marty puts on his hat and there's a POV shot where we see the train passing by some abandoned tracks on the grass to the left. Then a wide angle shows some explosions going on and the train passing by the same place again. (01:41:37)
Continuity mistake: After the train crashes against the "Tracks end" sign, the editing is all wrong: Marty is inside the car covering his head with the hover board. Half a second later he has magically moved outside the car, dangling and holding the hover board. Then we see Clara upside down, and back to Marty back inside, removing the hover board from his head after the crash. then he dangles outside with the hover board. So the first shot of him dangling shouldn't have been there. (01:40:42)
Continuity mistake: Doc warns Marty to watch out for the "Track ends" sign which can be seen just 10 meters away. Despite that short distance, the action goes on for several seconds. Given their speed, they should have crashed almost immediately after Doc's warning.
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.
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.
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 ends, Marty and Jen are blown away. They're either landing on shadowy or sunny grass, depending on the angle. This changes back and forth. (01:49:05)
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: When the movie is about to end, Marty picks up Doc's photo. From the front shot he is grabbing it by the middle, but in the opposite angle he holds it by the lower end. (01:48:40)
Continuity mistake: After the breakfast machine dumps the eggs in the pot at the start, the eggshells disappear.
Continuity mistake: When Marty walks around the old town and watches the manure guy work, a huge shadow covers the whole place. An instant cut later, it's all sunny, revealing it was shot at a different time of day.
Continuity mistake: After Marty gets to 1885, a bunch of Indians are running to him and the cavalry behind them, but in the open view one second before, the cavalry was not there.
Continuity mistake: In 1955 while the Doc is recording some stuff, while Marty behind him is sleeping on the couch, his head moves from side to side between shots.
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