Other mistake: When McNulty uses the stopwatch to rob a bank, he goes through a small wooden door and past the employees. With time frozen, the small door should have stayed open but immediately swings shut.
Continuity mistake: After he's dragged behind the horse, MacKenzie's facial abrasions keep changing completely from one scene to the next. And one day later, they've miraculously healed enough to be covered by one little bandage.
Factual error: The series takes place during the 1930s prohibition era. The $50 bill projected on screen is series 1953A with signatures of Ivy Baker Priest and Robert Anderson.