Other mistake: Van Helsing's ring fits his as a human, but in the climactic fight he turns to werewolf form and back and is still wearing it. He didn't have time to take it off and put it back on, so either one of his wolf fingers should be horribly, painfully cramped or the ring should have broken. No one insinuates that the ring has magical powers of any kind to prevent this.
Other mistake: When there is a werewolf transformation, the infected person's skin falls off to reveal the werewolf underneath. When changing back to human, the werewolf's skin falls off. When Van Helsing becomes a werewolf, his human skin falls off as he changes. As he battles Dracula as a werewolf, he grabs Dracula's throat. As soon as he does, the full moon is covered by the clouds and Van Helsing simply reverts to his human form. No werewolf skin falls off, he just changes back.
Other mistake: When Igor tortures the Werewolf behind the canvas, both of them can be seen as shadows, meaning a source of light is creating these images. When Igor hits the Werewolf with his staff, sparks can be seen through the canvas. First, as the image is a projection, the sparks would appear somewhere else on the canvas, and not exactly on the position of the staff's projected image. Even if the images are just silhouettes, the angle of the shot and the visible pillars indicate a much larger size then Igor really is. Second, such an intense source of light would alter the projection of the shadows a lot, not just a little bit in front of the Werewolf. In the case of silhouettes, they would completely disappear, as the source of light is between the 2 actors, and not at the side of them.