Factual error: The problem with firing a cannon on steam power is to have the boiling water build enough pressure inside the barrel, with no leaks or openings to release it before its time. The professor's scarf and the plug hammered into the fuse hole (which also flies off just before the shot) wouldn't be good enough as pressure restrainers.
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The Fearless Vampire Killers mistakes
Revealing: It is obvious that the night scenes were filmed in broad daylight, since clear shadows are still cast on any available surface.
Continuity: In the final sequence, when Sharon Tate awakes from death as a vampire, she rises and sits straight up; but in the next shot, when she buries her teeth into Polanski's neck, she leans back in the sled's seat before attacking him.
The Fearless Vampire Killers trivia
The movie went by quite a number of different titles: 'Dance Of The Vampires' (original UK title); 'The Fearless Vampire Killers, Or: Pardon Me, But Your Teeth Are In My Neck' (US title); 'Vampire Ball' or 'The Vampire Killers' (working titles).
It was during the casting tryouts for the movie that Polanski and Tate first met; they got married about two years later.