Factual error: Selene's guns are apparently capable of firing 30 rounds without reloading. Considering the size of the ammunition, this is physically impossible.
Continuity mistake: At the shooting range, Selene is shown firing a black USP without a compensator. It then cuts to an over the shoulder shot, and she is now firing a two-tone USP Match (which she supposedly receives from Kahn to test out a few seconds later). It then cuts to a frontal shot of her, and she is once again using the same black USP. She never stopped firing, so she couldn't have switched guns.
Continuity mistake: Selene shot Singe in the right shoulder, but when she brings him in front of Viktor, the injury is in the left shoulder.
Continuity mistake: Whenever Raze transforms into his wolf form he takes off his jacket, but when he leads the other werewolves to assassinate the council members and to drain Amelia of her blood he all of a sudden has his jacket on. He couldn't have stored it on the train.
Continuity mistake: In the last fighting scene between Victor and Michael, Victor's face gets scratched and it starts bleeding. In the next shot, his face is completely cleared of his scratches. And in the next, the scratches appear again.
Chosen answer: In most of the shots, Lucien takes his coat off when he changes to wolf form, and the blade would be attatched to a mechanism in him coat. Also, the film gives us an example of the blade hurting a vampire: bloodloss. And that kind of weapon could do some serious damage, and as it probably wouldn't kill the vampires, it would almost definitely slow them down.