Character mistake: On the day Rebecca arrives home, Junior Douglas says that he was "Valedictorian, Class of '95." Earlier, Drago talked of almost killing Running Buffalo "during that trouble back in the '40's," which would have been about 50 years earlier. That would mean Drago and McLintock were fighting Indians when they were babies, or possibly before they were born!
Continuity mistake: In one of the funnier scenes of the movie, a tipsy John Wayne attempts to escort Maureen O'Hara up a flight of stairs while drinking from a bottle of whiskey. They fall to the foot of the stairs, and the bottle is empty. After bopping Wayne on the head with the bottle, O'Hara throws it against the wall, and it's miraculously full again.