When Deacon leaves to pick up a body, it goes from a quite bright evening to quite dark in between him getting in the car and then driving away from the house. When he goes to fill up petrol just a few minutes later it's fully dark.
The scar on Anna's forehead is at times quite smooth and clean, such as when she is lying on the gurney. But when Deacon has her looking in the mirror, and the camera goes back and forth, it appears as a very rough, uneven cut, even with the stitches. Then once again it becomes smooth and clean. Apparently these were from different takes and resulted from different makeup staff.
When Deacon comes into the morgue the very first time, the door opens to the inside. When he leaves after a while, the door opens to the outside. During the rest of the movie the door opens to the inside.
(at around 1h 11 mins) The Ohio Bicentennial license plate on Paul Coleman's car is JB751C. Ohio Bicentennial plates were only issued in 2 letter, 2 number, 2 letter combinations. A plate in this sequence would have never been made.
Eliot shoots his victims with a folding packfilm Polaroid, yet the pictures in his wall are integral 600 film - fake 600 integral film. 600 film is black on the back, and when he is looking at Anna's picture the back of the picture is white.
The goof items below may give away important plot points.
There are at least two vans in the movie, one a Ford Econoline with small porthole windows at the rear and the other a Chevrolet Express with no windows. We see the Ford most often in long shots, it was the one in which Deacon harasses Anna before her accident. He later loads a gurney into the Chevy but drives away in the Ford. The Ford pulls up to the gas pumps but it's the Chevy that Deacon begins fueling, you can even see the Chevy "bowtie" symbol on the back door followed by the word "Express". At the end of the movie Deacon and Jack are sitting in the Chevy as the ambulance passes.