Revealing mistake: When Montoya uses a spear to toss the vampire woman from the prison door into the sunlit street, it is an obvious dummy that has been used. First you see the woman stumble out, and the first flames shoot out of her, then the angle changes to a close-up of her as she is engulfed in fire. In the close-up, she has become bigger and rounder, and does not move.
Plot hole: In the scene where Katrina is watching through Valek's eyes as he kills father Molina, Crow orders father Giteau to "find out where an old padre is missing". But Katrina never described father Molina, all she said was "He killed the priest". Crow could not have known the age of the man who was killed.
Continuity mistake: In the first part of the movie, when Jack and Montoya returned to the Sun God Motel for burning their friends, there is a wound in Jack's neck. But in the next shot it is not there.
Other mistake: When Valek and the other master vampires arise from under the ground, which is supposed to be after the sun has set, shadows are cast on their faces from a light directly above them.