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: In the middle of the desert banquet scene, sand is lifted off the ground a second before the genie belches. (00:29:40)
Revealing mistake: When the zombie crawls into the hardware store breaking through the bottom part of the barricade, you can already see on the wood plank the line where the wood is supposed to snap in half. (01:05:20)
Revealing mistake: There's definitely more than one cat playing "Panda", but how many can't be determined because Panda's entire body is not shown at once. "All" of them (maybe a whole litter?) have various distinctive black spots on a mostly white body. Watch for the amount of black on a cat's face and whether or not the diagonal black fur on the right side covers the entire eye or only half and if there's a black spot toward the nose. Also, look for how much black is on its side, back, behind its legs, etc.
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.