When Rex and Tanith are riding in the green convertible sports car, there is a part where Mocata's face appears in the rear-view mirror and "communicates" with Tanith. In close-up, this mirror is shown to be mounted on the lower center of the windshield, but in subsequent shots from different angles, no such rear-view mirror can be seen anywhere on the center windshield.
When Rex arrives at the forest to witness the Black Mass, it is almost totally dark. Later in the evening when he phones de Richleau, however, the view outside de Richleau's window shows it is broad daylight.
From after Rex has fallen asleep at Marie's house (when he has removed his tie), his shirt alternates between having one and then two buttons undone in various shots. Notable is the final scene: when he is walking up the garden, there is only one button undone. When he enters the house, two buttons are undone.
During the opening credits, a symbol is shown that is not Satanic, and it is also incorporated into the symbol on the priestly robes during the film. Inscribed within a Star of David, there is a six-winged seraph with the faces of a man, lion, ox and eagle based on the vision of Ezechiel. In Christian tradition, the four faces become associated with the four gospel writers: Matthew, Mark, Luke and John.