Continuity mistake: After Marcus kills Kraven, we cut back to Selene and Michael. As he talks about her saving his life and that how everything has changed, part of Selene's fringe switches from hanging in front of her face to tucked to the side between shots. In the closeup shots it hangs in front of her face and in the wide shots it is tucked to the side.
Continuity mistake: In the scene where the helicopter is outside the castle, the entry team dons their diving gear to enter the water. They put on gas masks with one viewing panel. When the shot changes to a water view the entry team is wearing scuba masks, which have their viewing panel divided in half by a black line. When the team emerges from the water within the castle in the next shot they are back to wearing masks with one window.
Continuity mistake: After the fight on the barge between Michael and Marcus, Selene tries to revive the impaled and seemingly dead Michael. At one point she rips his shirt open to pour her own blood into his wound, then the shot changes to her face, then changes again, slightly further out. It is in this shot, you can see Michael's shirt is still intact.
Continuity mistake: When Michael is being shot by the police, Selene can hear the shots very clearly, despite being down in the basement off the safe house. She goes out and goes towards the shots. Although she's much closer to the action, and outside the building now, the shots seem a lot further away.
Revealing mistake: As Selene and Michael infiltrate Tanis's lair of exile, Selene kills both of Tanis's vampire girlfriends. Afterwards, as Tanis is carrying one of the dead girls' body to a dungeon we can plainly see the girl's arm holding onto his shoulders/back of his neck. If she was dead, her arm should have simply hung somewhere - not draped around his shoulders, obviously holding on like she is.
Deliberate mistake: When Selene and Michael are driving a truck, chased by Marcus, at one point in the chase Marcus flies directly ahead of the truck and stays upright in the air with wings outstretched, facing the truck. Given that the truck is travelling at high speed and Marcus is using wings for flying, it is just not possible. It can't be attributed to Marcus' inertia - he stays in front of the truck for a couple of seconds, long enough to lose any speed advantage he had before.
Other mistake: When the glass in the Range Rover gets broken, the front window and passenger window shatters as normal automotive safety glass should. However, the driver's side glass gets broken into large jagged shards, instead of smaller pieces as would happen with any kind of auto safety glass, whether tempered, coated, or laminated.