Continuity mistake: When Doc's about to send Marty back to 1885 from 1955, and Marty's dressed up...when Doc's walking back and forth loading up the car - look at the shadow from the building. You see it changes by several feet in a few seconds as the scene was obviously shot throughout the day. (00:16:35)
Continuity mistake: When the Doc of 1955 is about to send Marty back to 1885, he turns the time circuits on. However, if you look at the time circuits after Marty has reversed, you can see that they are turned off. (00:17:15)
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: When Marty goes back from 1955 to 1885 and meets the Indians, note that none of the Indians shoots any arrows nor do any arrow hit the car. You can see that when he drives the car back into the cave. But when he gets back to the cave after the Cavalry ride, a long arrow sticks out on the side of the car. (00:18:10 - 00:20:00)
Continuity mistake: When Marty arrives in 1885 and runs into the Indians, he comes to a slow rolling stop. But the digital speedometer on the dashboard shows that he is still traveling about 70 miles per hour. (00:18:55)
Continuity mistake: Watch the scene with the Indians (just as Marty sticks the DeLorean into reverse) and see the differences of how close and far the Indians are from the DeLorean. (00:18:59)
Continuity mistake: When Marty is being chased by the Indians, a wide shot with the DeLorean at the bottom of the screen coming towards us and the Indians following reveals the entire background for a mile or two. However, there are no cavalry visible anywhere despite the fact that they pass over the cave in the following shot only five seconds after the last Indian does. (00:19:15 - 00:20:20)
Continuity mistake: When Marty is inside the cave (where he hides the DeLorean), keep a close eye on the huge landscape and very large mountains behind Marty when he is screaming at the bear. The entire background changes when he runs out of the cave to just a few hundred yards of ground before it drops down, where Marty falls into the fence. What happened to the vast landscape and mountains? (00:20:30)
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 holds the baby, the gown covers his legs. In the close-up the gown is up and the legs are bare. (00:26:03)
Continuity mistake: After visiting Seamus, Marty walks to the Hill Valley train station where a man is by the ticket window. In the next shot the man has disappeared. (00:26:35)
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 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 Mad Dog is hanging Marty outside the new courthouse, Doc takes aim stood in bright sunlight (when the camera is behind him), but immediately after shooting him down we see Doc from the front, stood in a large shadow. (00:33:48)
Continuity mistake: When Doc and Marty first see each other in 1885, just before they hug, Doc puts his hand on Marty's shoulder, in the next shot it's on his arm. (00:35:15)
Continuity mistake: When the DeLorean is being pulled by horses, the clouds in the sky change in the next shot. (00:38:50)
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: 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: While at the dance Marty is showing off at the shooting gallery. After shooting all the targets he twirls the gun and hands it back to the booth operator with the barrel in his left hand. Very the next frame shows the gun in his right hand. (00:54:28)
Continuity mistake: When Doc realises that his name has disappeared from the tombstone, the date on the tombstone remains, but in the next shot, it disappears. (01:07:47)
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