Continuity mistake: When Gary is remembering at the start of the film the incident with his family and is looking at the picture in the mirror, you can see there are 4 people in it. The next shot when Gary is speaking with Spottswoode the picture (to the left) has changed to being just 2 people in the picture.
Continuity mistake: At the end, when Dr Calvin, Del, and Sonny are in Robertson's office, when Sonny starts to trick Viki, Calvin has her body side on, then she is completely facing Sonny, then back to side on, then finally back to facing Sonny again. (01:27:55)
Continuity mistake: When Patience Phillips wakes up after robbing the jewelry shop, her hand goes up to her bed and there are a bunch of jewels. She has a ring on that hand, however, as she gets a bag and puts everything in it, the ring is gone. She then finds that ring mixed in with everything, and tries it on the left and then right hand, where it stays. In the next scene as she keeps putting stuff into the bag and then finds the necklace, when she holds it up, her hands are visible... guess what? NO RING.
Continuity mistake: When Hellboy is walking at one point, holding his gun up in his left, gloved hand (just after one of the agents has said "This place is like swiss cheese"), watch the frame. It switches suddenly so his right hand (the stone one) is his left, and he's holding the gun in his "right." (00:59:40)
Other mistake: When Rebecca uses her computer to record movements in the greenhouse, the footage on its screen does not match with reality. (00:44:27)
Revealing mistake: After the El Dorado hits Boone's car and is just about to drag it, there is a quick glimpse of its front license plate. The letters are reversed, indicating that it is a flipped shot. (00:28:20)
Continuity mistake: When Hiram, Juan and Kelly first see Old Fred's wagon in the distance, the white tarp on it is draped across it and down one side. When they arrive at it, it's draped differently. They also arrive at it from the side, when they were approaching from straight in front on it. (00:46:15)
Revealing mistake: In the Arena Challenge scene, the statue of Toa Lhikan falls on the herd of Vahki and part of the statue's mask is going through the ground. (00:22:30)