It's a simple fact of cinema that movies are going to end up with some mistakes making it into the final cut.
No matter how obsessively perfectionist a filmmaker might be, there's no way for any single person to keep track of every last variable that goes into completing a film.
For the most part, audiences are happy not to fixate on the minor details if a film is good enough, though when films reach a certain level of popularity there's an undeniably rabid breed of fans who will pick out every last peculiar mistake.
But sometimes there's more to mistakes than mere sloppiness, and sometimes there's compelling evidence - if not outright confirmation - that the gaffes were intentionally kept in the final cut after being spotted, or perhaps even partially conceived by the filmmakers themselves.
These 10 horror films, many of them classics of the genre, each contain an eccentric mistake which, in the end, was left in the movie to further the film's off-kilter tone and keep audiences on an uneven keel.
The fact that many of these "mistakes" are still being fervently debated today is a testament to how utterly ingenious they are...
10. The Visible Crew Shadow - The Amityville Horror
Perhaps the most iconic moment in the original Amityville Horror is the bone-chilling sequence where Father Delaney (Rod Steiger) attempts to bless the Lutz family's haunted new home.
But Delaney quickly finds himself facing off against a bothersome swarm of flies, and just as he's about to double over in pain, the door opens and a voice whispers, "Get out," before then screaming the utterance at him.
It's a sublimely creepy scene on its own merits, but one which is elevated in a subtle way by a small mistake.
Moments before the door opens, we see a dark shadow briefly glide over the top of the frame and then disappearing from sight (2:35 in the below clip).
This is very clearly a piece of camera equipment or a crew member whose shadow accidentally got into shot, and yet the ambiguity of the end result is offputting enough to fit perfectly within a haunted house movie. And so, director Stuart Rosenberg evidently kept it in.