Continuity: 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.
Revealing mistakes: In the observatory, when Simon lands on the head of the Goat of Mendes after being punched by de Richleau, it is clear the floor is made of foam rubber in order to cushion his fall.