A younger stunt double takes over the fall midway down at approximately 36:00. As she "lands" at the bottom of the stairs she appears to be crawling into position before she comes to a full stop. Her resting position changes between shots. First her skirts are up with petticoats showing. In the next shot her legs have moved and her skirts are now discreetly at her ankles. No character touched her in between.
The ghostly hand is a right hand throughout the film but when it appears to kill Mrs Luke it is suddenly a left hand.
Movie has trouble keeping track of the time of day. Often it is said that it's night, but when people go outside, it's not night anymore and the scenes are clearly shot during the day, probably due to how much easier was it to shoot them during the day.
When Charles finds an open window in Catherine's room and her bed empty, he walks towards the camera and is visibly squinting, probably due to studio light troubling him.
The movie takes place in 1795, and you can see that, while she sure can scream, Stephanie Beacham have amalgam fillings in her teethes - something that they started to use in France in about 50 years later.