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.
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 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 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