Revealing mistake: If you get a slow motion bullet cam shot on a zombie wearing a helmet or hat, and it's a head shot, you can see the hat fly off when the bullet hits, but often you will see the layers of the hat pass through the zombie's head with no collision.
Revealing mistake: Cathedral of Resurrection: There is a room with a radio before the third Safe Room where the radio is constantly saying "All hope is lost! Lord... Please save our souls." If you look at the wall to the right of the desk with the radio, you can see separation from the wall to the wall next to it, and the floor, where light is shining through from the other side. But it's not supposed to do that.
