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: Doc comes back from the past. Jennifer walks to stands next to him. In the following shot, she is still approaching Doc to give him the paper from 2015. (01:50:05)
Continuity mistake: At the end of the movie, when Doc and his family leave in their newly built train, the neon lights that flash over the front of the train go from left to right. In the next shot, where the train actually rises from the ground, they go in the opposite direction. (01:51:05)
Continuity mistake: When Marty is about to go back to 1885, he starts driving and you see him pass the building: then when it shows the car's tires, you can see he passes by the same building again.
Continuity mistake: When McFly gets to the village in 1885, he walks beside the manure guy and the sun changes between shots.
Continuity mistake: Just when Marty parks inside the cave to hide from the Native Americans, one wearing a war bonnet jumps over the mount. When the angle changes, the man is gone and has been replaced by two riders with no war bonnets.
Continuity mistake: When Marty is about to go to 1885, before Doc shoots his gun, you can see the coils on the face of the DeLorean is brown, later when he is driving toward the screen, the coils are clear silver again.
Continuity mistake: In the saloon, Mad Dog shoots at Marty, forcing him to dance. Marty goes into his moon walk dance, and just before he launches the spit jug at Mad Dog, he does a jump. Watch as he lands - the jug is suddenly 4 to 6 feet to the right of him.
Continuity mistake: When Marty is about to race against Needles, he holds the wheel with both hands. An instant cut later, his hands are gone.
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: When Marty is about to race against needles, the camera zooms in towards his car. Notice a big rectangular black object reflected on the car. From opposite angles it disappears and there's just green fields. The black object was most surely a structure to hide the camera equipment.
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.
Continuity mistake: While Marty is outside the mine, the sun changes on the floor.
Continuity mistake: While Doc saves Marty from Biff the sun changes between shots.
Continuity mistake: When Marty holds the temperature-meter and warns Doc to put the yellow log inside the firebox, his finger suddenly disappears between shots.
Continuity mistake: When Buford hangs Marty, he puts the rope around his neck once. In the close-up the rope goes around twice.
Continuity mistake: When Marty and Buford duel, Seamus' distance to the bartender keeps increasing / decreasing randomly between shots.
Continuity mistake: After the festival, Marty wakes up and grabs the gun belt, which is missing a bullet. When he mimics in front of the mirror the bullet has magically appeared.
Continuity mistake: When Doc holds the telescope and tells Clara that he'll fix it, right when they date at the festival the position of the telescope changes between the front and back 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