Plot hole: When Andy turns into a werewolf, he kills both Amy and Detective Benbou. Later in the morgue, Amy tells him because of this that she is forced to become his "rotting sidekick." If this is completely true, then Detective Benbou should also be following Andy around but he isn't seen after Andy manages to escape from the morgue.
Continuity mistake: Much of this movie was shot in Luxembourg, not in Paris. This is obvious on several occasions - the road signs are clearly not like the French standard. You even see a sign to »Trier (Trèves)«. In Paris, there would be no signs to this German town...and even if there was, these would say »Trèves«. The buses also have French registration plates but are from Luxembourg companies.
Other mistake: During the party in the church near the end the bad guy blocks the church door with a bulldozer. The girl climbs up to the top of the church, breaking off a stone gargoyle which lands on top of the bad guy while he's in the bulldozer. Then the girl climbs back down and gets into the bulldozer, which is completely undamaged from the gargoyle hitting it - the bad guy's body has completely vanished. She manages both climbs impossibly quickly.
Plot hole: Near the end of the film, at the big party in the abandoned church, one of the main characters is tied to a crucifix. In the ensuing mayhem, we see the crucifix fall backward, with the guy on top of it, facing the ceiling. In the shot where the panicked partygoers are fleeing, we see this guy running out of the church, carrying the crucifix, which is still tied to his wrists. So, how did he get up? And if someone stopped to help him, which seems unlikely, given the general panic, wouldn't it have been easier to simply untie him? The alternative would have been to hoist the whole kit n' kaboodle upright, which, at the least, would have taken two people.