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: Shortly before Sol's death, Thorn rushes into the suicide center and talks to him from the control room above. When Sol pleads Thorn to listen, there's interference on the intercom. Now watch the control panel: The earphone cable on the left side dangles back and forth each time the "speaking permitted" sign flashes on and off, blatantly revealing the cheap trick they used to make the display flashing - they simply toggled the "on" and "off" frames. (01:17:30)
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.