Revealing mistake: In the scene in the alley behind the museum where Hellboy first encounters Rasputin, Sammael has its tongue wrapped around Hellboy's arm. When Myers enters and shoots at Sammael, look closely at Myers' hands. Not only does the gun fire without him pulling the trigger, but the gun does not even recoil.
Revealing mistake: During the scene in which Rasputin is resurrected, at one point, Ilsa wipes some snow off a wall then checks her book to see if she can match a symbol. When she opens the book, some snow lands inside of it... and it is incredibly obvious that the snow is fake. It looks like pieces of torn up paper.TedStixon
Revealing mistake: When Kroenen cuts open the statue to get the Sammael container, the slash on the statue is actually at a different angle than he actually slashed it. Probably the actor missing the correct angle, which is admittedly difficult.