The Fearless Vampire Killers (1967) - 6 mistakes

Revealing: It is obvious that the night scenes were filmed in broad daylight, since clear shadows are still cast on any available surface.

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.

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.

Other: When Krolock ambushes Sarah in her bathtub, he bites her well above the place where the bite marks later show.

Factual error: The professor and Alfred chase the recently undead innkeeper into the cellar, where, thinking him cornered, stake a wine cask behind a curtain. Even with power behind the blow, a wooden stake cannot easily pierce a wine cask; they are built too sturdily for that.

Factual error: I don't know whether they had bubble baths in an isolated village of the late 18th/early 19th century; and even if they did, the foam in Sarah's tub looks remarkably untouched (except for the blood) - considering that she had just been trashing around in it when Krolock attacked her.