Character mistake: At the ravine, upon learning that the bridge doesn't exist yet, Marty states to Doc "We can't wait a year and a half for this to be built". If you carefully look at the sign, it says the bridge is opening Summer 1886. That's under a year away, not a year and a half from Sept. 1885.

Back to the Future Part III (1990)
1 character mistake
Directed by: Robert Zemeckis
Starring: Christopher Lloyd, Elizabeth Shue, Lea Thompson, Mary Steenburgen, Michael J. Fox, Thomas F. Wilson
Factual error: Though extremely modest on today's standards, the dress worn by Clara to the hoedown shows far too much cleavage for the time. No schoolteacher would ever wear a dress like that in the 1880s.
Trivia: Director Robert Zemeckis asked for Clint Eastwood's permission to use his name for BTTF 3. Clint Eastwood felt so honored that he said yes.
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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