Continuity mistake: When Marty is walking into the 1885 Hill Valley, he walks past an A. Jones manure hauler, which is embedded in shadow. In the very next shot, not only is he several more metres down from the hauler, the hauler is in sunlight. (00:27:31)
Continuity mistake: When Doc has just consumed the 'wake up juice' and runs out of the bar and dunks his head in the horse trough, Marty and the bartender come out after him. You can see that Doc is now soaked and a trail of water is on the ground behind him from a previous take of Marty dragging him back in the bar, which hasn't happened yet at this stage. Only really noticeable in the cropped version. (01:20:05)
Continuity mistake: When Doc and Marty are chasing the train on horseback, Doc gets on the train first and his horse pulls forward. Then when the camera shows Marty on his horse, another horse can be seen in front of him. When Marty jumps on the train, his horse falls back. But in the next shot you can still see the horse as it moves off camera. (01:30:55)
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 When the angle changes, the man is gone and has been replaced by two riders with no war bonnets.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: When Doc and Marty get on the train at the end, Marty has some trouble getting on, and note that his hat falls off in the sequence, but in the next shot when Doc and Marty fall the hat is back on. (01:31:15)
Continuity mistake: When the Doc enters the destination time into the time circuits when he is sending Marty to 1885 from 1955, the time/date etc. Go in as he's typing it. However, in the previous films and later in the 3rd film when traveling back to 1985, the enter button must be pressed before anything appears for the destination time. (00:17:25)
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 at the shooting gallery at the party and has his go, he points the gun in the wrong direction but still the targets go down. (00:54:18)
Continuity mistake: In 1885 Hill Valley, the hills are shown to be almost directly behind the courthouse, however in the previous films there was some distance between the two. This was likely due to the different shooting locations. (00:26:40)
Continuity mistake: When Doc and Marty are chasing the train, first their shadow is on the left of them, then in the next shot it's on the right. (01:30:40)
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: In the scene where Marty is driving the black Toyota pickup at the end of the movie, the truck's registration plate is missing - it can be seen when the truck is in the garage at the start of the film. Also the driver's door quarter light (vent window) is open. When we see the inside of the door it is closed. (01:41:45)
Continuity mistake: Just after Doc saves Clara from the ravine, the carriage flips on its side and rolls. In the next shot, it's stopped rolling and is falling neatly straight down. (00:41:30)
Continuity mistake: Watch as Seamus hands Marty the baby, there is a definite "jump" and Marty is facing in almost completely the wrong direction, looking for a clue from the set's crew as to where he should be looking to catch the baby from... (00:24:20)
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 Clara is about to leave Doc's workshop, the position of Doc's hand on the telescope alters each time we move from rear to front shot. (01:08:09)
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.
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