Revealing mistake: The outline of the blood pack is visible under John's shirt when Stillson's body guard shoots him. (01:37:35)
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: The stunt dummy that falls from the balcony in John's stead is pretty obvious. Its blank face is even visible briefly as it falls, and again when it lands. (01:37:40)
Continuity mistake: When Dodd's mother falls on the landing after being shot, the blood stain on the front of her green sweater disappears. (00:55:20)
Continuity mistake: When John tells Dr. Weizak about the dead zone, some of the items on the table change positions, and disappear/reappear between shots. (01:27:45)
Revealing mistake: When the murderer's mother is shot, the outline of the bloodpack under her clothes is visible. (00:55:05)
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)
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.
Visible crew/equipment: When Sarah drives away from the clinic in tears after seeing Johnny, the side of her car shows us two banks of studio lights, a reflector panel and part of the camera crew. (00:24:00)
