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 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.
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.