Other mistake: In one shot, during the lynching, the noose comes undone from the character's neck while he is hanging.
Deliberate mistake: In the scene where the FBI agents are transporting Lester Cowans in a car and telling him all his friends have confessed, for about three seconds the scenery goes by backwards. For some reason, the shot is played in reverse.
Continuity mistake: In the opening sequences where the Klan is after the civil rights kids, the Sheriff's car ('57 Chevrolet) doesn't have the red light beacon on top. The red light isn't seen until right before the civil rights workers' Ford station wagon is stopped.
Continuity mistake: During the lynching scene, the Dad wakes up his son and daylight is shining through the window. In the next scene when he runs out the door with his shotgun it is night time.