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)
Visible crew/equipment: In the saloon scene where Doc passes out, a production light with barn doors and stand can be seen in the upper left over Marty's shoulder.
Revealing mistake: When Marty and Shamus are sharing a meal at the dinner table, Shamus passes Marty the plate of rabbit. As Shamus hand moves away to let go of the plate, to the bottom right hand corner of the plate, a purple patch appears briefly where the two M. J. Fox actors interact, an error with the Vistaglide camera tech used back then. Easier to spot on Blu Ray Version, better than DVD. (00:24:10)
Continuity mistake: When Marty is shot by Buford, he is thrown off his feet and lands on his back. As he does, his poncho flips up, showing his shirt all the up to the middle of his chest. The stove door he is supposedly using as a bulletproof chestplate is conspicuously missing. (01:27:20)
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)
Visible crew/equipment: When Marty is with Jennifer in his new truck, as the camera cranes down to where they're talking, you can see a crewmember with a white shirt and some of the camera and crane. (01:46:10)
Other mistake: When the movie ends and Doc says, "It means your future hasn't been written yet," watch his son Verne behind. He looks towards the camera and seems unhappy, makes a beckoning gesture (presumably at the crew), and points at his crotch. Seemingly, the kid was having a hard time trying not to pee on himself. (01:50:39)
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: 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)
Visible crew/equipment: At the drive in theater, Marty says he'll put the boots on when he gets there. The camera pans to the left to follow Marty and Doc as they continue to talk. On the very far left side of the screen, the shadow of the boom mic briefly dips into frame. This happens just before Doc walks over to the DeLorean from his car. (00:16:35)
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: 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)
Visible crew/equipment: During the party scene, towards the end when Marty and Doc are discussing the duel and the fact the train may be late, you can briefly see a PA system center left of frame (bass cab with PA head on top and above it 2 tweeters side by side) it would make sense that a PA system be on set during the scenes so the band and dancers had a soundtrack to mime/dance to. (01:01:29)
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)
Revealing mistake: When Clara's dress rips off and is about to fall off the train, Doc saves her. One can notice it's not Christopher Lloyd but his stunt double. (01:41:17)
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 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: 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)
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