Deliberate mistake: There are absolutely no lights ever to be found in the underground world and the ceiling is literally the surface's ground, but there is still a perfect level of lighting in the underground world regardless.
Factual error: During the scene where Clint Eastwood is trying to convince Don Rickles to join in the heist, there is a box of Almond Joy candy bars behind Don's head. They weren't introduced until 1946.
Plot hole: At the end of the movie, the sheriff is going home and gives his deputy and the mayor a ride home. They pass over a bridge with a sign that says 'Leaving Colusa County'. Surely they don't live outside the county?