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.
Suggested correction: He didn't intend to stay in there but brought them just in case. A good policeman is prepared.
What, he wore his jammies under his uniform "just in case"? He isn't carrying an overnight bag.
Other mistake: The basement door of the Wadsworth's house closes itself after the Baby enters. Previous scenes show the door cannot close and shut itself.
Other mistake: When Karras throws himself out of the window, why is a man who needs a cane running full speed down those steep steps? It makes no sense since people use canes for balance.
Suggested correction: Some people have a cane as a fashion statement. Also, it's possible the cane helps but is not totally necessary, certainly able to be ignored when rushing to an emergency.