Revealing mistake: His tail disappears near the beginning of the movie. (00:10:55)
Revealing mistake: After impaling one of the Giants with the tree, when Hellboy is standing up, the seam of his costume is visible on his back. (00:34:20)
Revealing mistake: When the elevator begins to fall, Hellboy raises his left arm, revealing that David Harbour's arm is not painted red from the wrist down. (01:45:00)
Revealing mistake: In the beginning of the movie, when Hellboy is speaking on the phone with "Dad", he's speaking on on speaker, but when the phone shows up, instead of showing a screen of a call on speaker with the duration on the call, the screen shows what would appear if he was receiving a phone call before accepting or declining. (03:35:00)
Revealing mistake: When the elevator crashes down and Hellboy wakes up and starts to move, the prosthetics on his head move unnaturally around his horns and forehead as he gets up, revealing they aren't real.
Revealing mistake: When Hellboy is seen filing his horns towards the beginning, as he runs the file along them (right before Broom takes the file), the prosthetic horn stumps are moving up and down unnaturally for a moment. As they are meant to be protruding from his head, they shouldn't be moving up and down as he moves the file back and forth.
