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: The film was rushed to theaters, leaving a few mistakes which Universal fixed. In the original cut, toward the end, Judi Dench's human hand is visible, complete with her wedding ring. This was fixed for the re-release.
Revealing mistake: The signal sweeps shown on the 'orca' device and on other screens throughout the movie are all identical.
Revealing mistake: When Hank severs the head of the second zombie with the shears, the body staggers against the barricade, and one of the planks that is supposedly keeping away a throng of zombies from the HW store wobbles going noticeably out of place. (01:05:50)
Revealing mistake: Of the different cats playing "Panda", one that did not get along with the black cat was chosen for the fight - and the black cat was actually attacking Panda, who was hissing while in a defensive position (on its back, using front and rear claws to try to protect itself and get the black cat to stop). The safety of the two cats fighting was "sacrificed" for purposes of making the movie (a no-no!). A different cat was most likely used for the more playful pawing between Panda and the black cat. (00:04:33 - 00:05:28)
Revealing mistake: During the interview, you can see a monitor with the lyrics of "Something", even though he is supposedly improvising.
Suggested correction: This is a deleted scene. It is in the extras on the DVD but it is not part of the movie.
Revealing mistake: A basket with fireworks catches fire. When the basket is flung into the air, they forgot to add the CGI for it, so it's not burning at all. But the effect is there in the second shot, when Aladdin bicycle kicks it away. (00:06:00)