Continuity mistake: Henrietta Dodd shoots Johnny holding the gun in her right hand. Sheriff Bannermann emerges from the bathroom and shoots Henrietta. The film switches to slow motion as she is shot and stretches both of her arms out to the sides as she falls. When she lands, her left hand is resting next to the railing of the stairs and is covered in blood. At no time did her hand come in contact with her wound or get any blood splattered onto it.
Revealing mistake: When the murderer's mother is shot, the outline of the bloodpack under her clothes is visible. (00:55:05)
Continuity mistake: When Stillson is walking up to the stage for the final rally, Sarah is behind him holding her baby. The baby switches from left shoulder to right shoulder repeatedly.
Factual error: When the milk tanker detaches from the truck hauling all of the side lights remain lit. The lights are powered from the truck through cables that would definitely disconnect in this scenario. (00:08:10 - 00:09:00)