Continuity mistake: This is actually in a deleted scene on the DVD: Jones shoots a mountain lion at night, and it's seen to be completely dark out. The two other men go out to look at it, but when they're walking across the snow, the sky is suddenly pale blue and there's light just below the horizon. It looks like it's just before dawn or just after sunset. But when they reach the dead mountain lion, the sky is suddenly black again, and it looks like it's the middle of the night.
Factual error: In the final gunfight, we see a close-up of Charley fanning the hammer and rapid-firing his six shot Colt Single Action Army nine times in a row. [Kostner has stated in interviews that this occurs because a short scene of him switching revolvers (he had a second revolver in his belt) was left on the cutting room floor. Still a mistake, but there's the reason.]
Continuity mistake: As the girls ride in to town to avenge Kim, there is a deputy in a faded red shirt on a second-floor balcony. The first time he gets shot, he does a flip onto the street. The second time he gets shot, he goes over backwards.
Revealing mistake: Why does the railroad station look so old? It should be almost new at the time of the film.