Continuity mistake: Lucian has just walked into the werewolf fight right after the opening subway chase scene. His face is faintly lit in medium shots and completely in the shadows in closeups. (00:11:00)
Continuity mistake: After Selene has killed Viktor she approaches Michael from behind. We see him turning his head. When it cuts he turns his head again. (01:46:10)
Continuity mistake: When Michael was captured and hung up, and meets Lucian face to face, Lucian injects blood into him and the syringe has an air bubble in it. In the next shot, the bubble has gone and the syringe is entirely filled with blood when being injected into Michael. (01:23:00)
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.