When Gabriel and Izzy are at the dry cleaners, the computer screen at first is a bright blue color, then the next scene it is a dark blue, almost black color.
Hollywood insists that Franciscans are monks but the monastic habit is very distinct from the Franciscan-like habit seen being worn by the "monks" in this movie. A monastic habit has a scapular or an apron like garment and the robe is belted by a leather belt, not a rope cord seen. Also most monastic habits are black. Everything about the robes worn are Franciscan in design. This mistake could have been resolved by just calling them friars and calling the place St Gregory's Friary.
When Danyael is showing off for the kids with the chair, there's a shot of his head and neck. The collar of his shirt is lifted a couple of inches off his neck, obviously from the wire or harness he's using for that stunt.
Izzy is standing outside the church, gun in hand in a standoff with police. When Gabriel exits the church, Izzy quickly hands the gun to Gabriel, who is immediately met by a rain of police bullets. A number of exploding blood packets are clearly visible, almost extruding beneath Christopher Walken's shirt.
Spelling mistake: an early scene in the movie takes place at St Gregory's Monastery, but the carved plaque at the building's front gate spells it "Monestary".