The news reports that "all the bodies were decapitated" and that there were 19 total bodies. However, Jack clearly didn't decapitate the hookers, and there are obviously not 19 heads in the bags he buries.
When Jack hits the brakes of the truck irritated by priest's constant talking, priest falls on the side, but we don't see him getting up, and in the very next shot he is just sitting.
When Jack tips something over with his feet before opening the basement door at the beginning of the movie, position in which he is shown in the next shot doesn't match the one in the shot before.
When Katrina is telling Jack and Montoya where the Master is,
she screams "Oh, my God, he killed the priest!" Montoya asks how they were going to find which church in San Miguel, Jack says "The one that's missing an old guy". Katrina never said if the priest was young or old.
When an arrow is shot in to a cross on to which Jack is tied, Jack is holding his head on the opposite side from what was shown in previous shot.
Regarding when Anthony Montoya (Daniel Baldwin's character) fires a short burst through the sub machine-gun and then uses the barrel to cauterize his neck wounds; there's no way such a short burst would produce the kind of heat needed to cauterize a wound like that.
Frank Darabont is listed in the credits of this film as "Man with Buick" when in fact the car that was stolen from him was obviously a Cadillic.
When Katrina fully vamps out, she bites and tears a chunk out of the side of Montoya's neck, which would have torn open major blood vessels and caused him to bleed out. Not only does he survive this wound, but when he wakes up and acts to cauterize it there is very little blood down the side of his neck and the wound itself has already stopped bleeding.
When Jack spits at cardinal, we see cardinal clearing his brow as like Jack spitted there, but there is no spit.
Montoya attempts to cauterize a neck wound using the heated barrel of a sub machine gun. Although the special effects are shown (vapors and a sizzling sound suggesting heated flesh), it can clearly be seen that he never actually touches the weapon to his neck.
As Montoya rescues Crow from the cross he shoots into the front of the cross with an arrow, this implies that this side of the cross would crash to earth first (with Crow facing earthwards), but the backside of the cross lands in the next shot on earth.
At no time is it ever mentioned or suggested that a vampire's fangs are retractable. However, at several times (such as when Valek says "you are alone, crusader") it is clearly seen that Valek's teeth are normal.
During the fight with the masters at the Spanish mission, one of the monks goes flying into a ladder against the wall. The harness he is wearing can be seen when his robe flies up.
During the escape from the Sun God Motel, the chroma-key green screen can be seen outside Jack Crow's side of the pick-up truck before they wreck into the abandoned trailer.
Towards the end of the movie the vampires are in an abandoned and condemned prison. The movie shows a sign clearly stating; Condemned. No Trespassing. The slayers simply slide aside one latch and walk right in, then when inside there is electrical power and the full video surveillance system and elevator are fully operational.
When Katrina says the Master killed a priest, Jack tells Guiteau to call all the churches about a missing "old padre", but she had said nothing about the priest's age.
After the first hunt, when the vampire skulls are lined up on the hood of the Jeep, Catlin is counting them, exclaiming, "Nine! Goons, man! Not a bad day's work." Then Montoya comes up behind him, pats him on the shoulder, and says, "You're the man, Boone."