Continuity mistake: In the House of Erebus, right before Blade jumps off the ledge to fight Frost, you see that Karen is standing on the ledge above everything and Mercury is standing down below on the platform. But when Karen's shooting at her, Mercury is coming down the stairs from above.
Continuity mistake: When Blade gets back from the park after meeting with Frost and getting serum, he discovers Frost's men have been there, and sees Whistler. When Blade moves the tape, which is in a metal rotating stand, the stand rotates on its swivel, but in another shot it has only moved a little bit and hasn't rotated either. The tape would've been facing the other direction because the stand swivelled, but instead it remains in the same position.
Continuity mistake: When Blade drops Karen off, she walks to her apartment and as she approaches the elevator a blonde woman walks up to its doors and stops, but in the following shot the blonde woman is now just coming into view and then stands behind the black woman instead of at the elevator doors.
Continuity mistake: When Blade and Karen are in the room of manuscripts, Quinn comes in with a load of guys. When one of the guys loses a hand by holding Blade's sword, there's blood all over the handle of the sword, yet, the next time we see the sword, there's no blood at all.
Continuity mistake: Frost says near the end of the film that the spirits of the twelve will awaken the Blood God. Fair enough, but when the prophecy is first translated, you hear twenty beeps as pictures of people pop up around the circle, implying that there are twenty people standing around it.