Revealing mistake: In some extended edition cuts of the film, Regan backwards spider walks down a flight of stairs, and blood falls from her mouth. But her face isn't that of Linda Blair, but of a stunt double.
Other mistake: During his conversation with the landlord of the pub he ends up staying in, Sergeant Howie makes it clear that he did not intend to stay on Summerisle overnight, that he had been delayed and so needed accommodation. Makes you wonder why he packed his pyjamas. He's wearing them when Britt Ecklund does her famous naked song and dance routine, and they are not new so we know he didn't buy them that day. When he arrived he didn't even think he'd be on Summerisle for more than a few hours - we don't see him with so much as an overnight bag.
Continuity mistake: The hot knives that are set up in the King Lear scene towards the end of the film are sometimes smoking and sometimes not depending on the shot.
Revealing mistake: Right at the end when John is dying look at the texture of his blood. It is way too thin, almost like coloured water.
Continuity mistake: The driver of a carriage is racing along a road during a rain storm. In long shot, it is pouring buckets & buckets of water. Close-up of the driver & it is a light rain. Back to long shot, buckets again. Close-up, light rain.
Revealing mistake: The eyes of Germaine's corpse shut, then open as Mrs. Wadsworth pulls the body off the bannister.
Continuity mistake: At the gas station, Arletty looks at the attendant discharging his gun in the nearby trees. In the next shots, she is holding her bag randomly with or without the shoulder strap up. (00:05:10)
Visible crew/equipment: When the camera zooms in on Sebastian as he is suspended dead by the snake charmer's rope, a visible wire can be seen suspending him in the air. (00:47:25)