Deliberate mistake: When Hellboy is attacked by the spider-lady, as the spiders begin to descend on him to steal the sword, you can easily tell that some of the shots of the spiders coming down are repeated 2 or even 3 times, and simply flopped horizontally to appear different.
Hellboy Animated: Sword of Storms (2006)
1 deliberate mistake - chronological order
Directed by: Tad Stones, Phil Weinstein
Starring: Ron Perlman, Selma Blair, Doug Jones, Peri Gilpin
Genres: Action, Adventure, Animated, Fantasy, Horror, Sci-fi
Continuity mistake: After Hellboy defeats the semi-vampiric heads, the Professor is seen destroying a small statue on his scroll representing them. Despite being destroyed, this statue reappears later in the movie.
Trivia: The scene where Hellboy battles the vampiric heads is taken directly from one of the stories from the comic book, entitled "Heads." It was faithfully recreated when Mike Mignola and the writers realized that the story would be perfect for the semi-episodic nature of the film, and the Japanese-themed tone.