When Mother Firefly pulls the pistol on Lt. Wydell the revolver seems very large, and the hammer has been cocked back. When she fires, the hammer is already down and does not drop. The gun also appears to be a different, smaller weapon when the scene cuts to Wydell being struck and killed.
When the side window of the car driven by the four victims is broken, it's regular pane glass, yet cars of that age use tempered glass. Later scenes of the same car with broken windows also show broken tempered glass.
The goof item below may give away important plot points.
When Bill is shown getting his right arm cut off, the ax hits the obviously fake arm and turns sideways. It does not go all the way through, but the ax wielder is shown immediately afterwards holding the neatly severed limb after only the initial swing.