Near the final scenes, the doctor falls through the stairs and lands on the circular saw, which gets covered in blood. Immediately after Ben covers the doctor, the camera pans up to reveal the perfectly clean saw blade in the background.
When Matt confronts Mike Ryerson in his house, Mike's autopsy scars change sides from right to left and back again.
When Ben, Mark, Father Callahan, and Dr. Cody find Mike Ryerson in the basement of the Marsden house, Ben asks Mark to hand him a stake. He takes the stake with his left hand and is facing Fr. Callahan. In the next shot we see Dr. Cody checking Mike's pulse and behind him Ben is holding a flashlight in his left hand, with nothing in his right, and he is turned toward the doctor. The scene then cuts back onto Ben, who again has the stake.
When Richard Straker confronts Susan Norton and Mark Petrie in Straker's house for the first time, Straker takes Norton's gun with his left hand, then Petrie's stake with his right hand. The stake then disappears and reappears in Straker's hand between shots.
In the final scene in the hospital, the doctor throws the orange hat and we see that it lands on the floor. In the long shot that ends that scene, the hat is on top of a pillow.
In some outdoor night scenes crickets and other insects can be heard clearly yet the ground is covered in snow. During Maine winters there are no insects audible at night.
The action takes place in Maine during winter but in many scenes there are trees such as birches visible with full green foliage. In Maine at that time of the year those trees would have shed their leaves months prior and would appear bare.
The film underplays something that is extremely dangerous. Ben douses the stairs in gasoline, then lights a match and throws it. The fuel ignites as if it were nothing stronger than kerosene, which it actually is. If it were gasoline, the fumes would have exploded like a bomb. If an audience member were to imitate this it could be lethal.
When they are in the church, Father Callahan is filling bottles with holy water. He fills one, fills the next and then fills the first one again.