Revealing mistake: When the spear handle is protruding from Hector's shoulder, the area of the armor around the circular wound is clearly visible and it is in perfect condition. There is no tear in the armor whatsoever, around the wood, which would have been necessary to allow the wide spear tip to penetrate the armor and actually enter Hector's body as it did. (02:01:20)Super Grover
Revealing mistake: After Helen finds the hair on Bob's suit, she goes to him while he is in the car and says," Have a great trip. I love you, so much.", Bob backs the car out of the driveway. He backs out and turns, but you can see his hand on the top of the steering wheel doesn't move. (00:47:05)
Revealing mistake: Very near the end of the movie, when the NOAA director is viewed in the helicopter as it approaches NYC, the image is reversed, verified by the David Clark logo on his headset being reversed. There are two different shots of this just a few seconds apart, and they are both reversed.
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: In the jewellery store when the robbers are smashing the display cases they shatter before being hit with the shotgun butts.
Revealing mistake: When the demonstration crowd is assembling in the plaza, there's a yellow tram entering on the left hand side. Watch as someone walks across the tracks in front of it and disappears through the corner of the cab. The shot must be a composite of the crowd amassing and the tram shot separately and then overlayed.
Revealing mistake: Nearly at the end, the scientists find Alice's body at the waterfall, one of the men lifts up the cloth to take a look at her face. Her face is badly burned on one side and she is pronounced dead. However, take a look at the left side of her neck, you can see her either gulp or take in a quick breath of air. (01:19:05)
Revealing mistake: When the musicians stop playing and the world leaders start getting up from the table, it goes to a shot spanning the entire table, but also showing the musician's stage. This is where you see the mistake, as you see Berkhamp suddenly at the front of the stage wearing his headband and dancing, when it cuts away from that shot, it shows Berkhamp is still sitting behind his instrument with his headband still off. Then we find that out of place shot was recycled from when Berkhamp actually was onstage and dancing as we see the exact same shot again less than a minute later, this time the shot is where it's supposed to be however this is still a mistake and quite an odd one. (01:26:25)
Revealing mistake: At the end of the movie, when the last "Graboid" appears, in one or two shots of him retracting back into the ground, you can tell the shots are reversed, as dirt is being sucked down in an unnatural way. It flies up and sort of "implodes" towards the creature. (01:23:50)