Deliberate mistake: When Blade Abigail and Hannibal go to visit Dr Vance, Drake grabs Hannibal they move to a window, if you look behind them, the window is closed, then when Drake stabs Hannibal the window is open, then after drake goes out the window, you hear the window smash, even though he went out an open window. (00:56:15)
Blade: Trinity (2004)
1 deliberate mistake
Directed by: David S. Goyer
Starring: Ryan Reynolds, Wesley Snipes, Jessica Biel, Kris Kristofferson, Parker Posey
Factual error: When Blade notices the piece of Dracula's armour in the Nightstalkers' headquarters he's shown an image of what the full armour looks like. It'd be impossible to extrapolate a full suit of armour from such a small piece.
More quotes from Blade: Trinity
Trivia: Ryan Reynolds gained 11kg (or roughly 24lbs) of muscle for his role as Hannibal King.
Answer: It's possible that he followed one of them back to the base. This is feasible since they wouldn't be able to tell that he was following them because of his shape-shifting abilities.