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Continuity mistake: When Marty returns to 1985, he goes to his house and gets in his Toyota pick-up. Look closely behind his head when he first gets in the truck, there is no driver-side head rest. But when he picks up Jennifer, when they pull up to the stop sign the head rest is there. (01:45:00)
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