Revealing mistakes: The sidewalk that Mary lands on near the end of the film visibly vibrates when she hits it.
Factual errors: Lafayette Cemetery closes at dusk (or maybe noon, reports vary); the gates are closed and locked. Yet we see the gates stand wide open very late at night.
Crew or equipment visible: When Dracula throws Simon across the room in the library, you can see the cable attached to him.
Continuity: The opening sequence (set pre-2000) contains a shot of London, including the Houses of Parliament, with a black-roofed building opposite (which, in fact, was not built until 2000). Later during the movie, now set in the present day, a similar shot correctly shows this building under construction, with bare foundations.
Revealing mistakes: When Lucy (Vitamin C) grabs Mary (Justine Waddell) her contact lens moves out of place.
Factual errors: Van Helsing's office building is in London, but the Security Guards on duty are armed. Security guards in England do not, and cannot, carry guns. Handguns are illegal in the UK and private citizens, even security guards, cannot even possess handguns, much less can they carry them about. Handguns can only be carried by the armed forces and by specially trained police officers (and even then it is rare to see even an armed police officer).
Revealing mistakes: When Nightshade accidentally cuts his finger with the chisel, there is visibly no blood on the coffin. Yet it shows the coffin sucking in the blood 'from his finger.'