Continuity mistake: When Cooper is at his desk and puts his head in his hands he is not wearing a badge. Then the kid comes in, and he looks up and the badge is there.
Other mistake: When Kane comes to warn Ms Ramirez that Frank Miller is in town, Kane puts his black hat in a chair by the door. When he approaches Ms Ramirez, he has the hat in his left hand.
Continuity mistake: Lee van Cleef's guns are turned backwards in their holsters when Sheb Wooley breaks the store window.