Factual error: As Anne is standing in a room at the beginning of her punishment, the camera pans sideways from her past a piece of furniture with a small statue on the top of it. From behind the statue comes a light whose color and quality could only be electric in origin. It is a greenish fan of light, like one of the old "TV back-lights".
Other mistake: When Ann makes fun of the mute at the dinner table, the mute wears a plum-burgundy dress. Later on when Grace runs down to the gate and hears Ann crying, she's wearing the exact same dress. Seems unlikely the woman of the house would wear the same clothes as the scullery girl.
Continuity mistake: When Mrs. Bertha Mills gives Grace her pills, she first sets them on the table, and we can see that the pills are about 1/2 inch apart. Then it cuts to a closer view, and the pills can be seen to be touching each other. Then it cuts back, and the pills are apart again.
Other mistake: When Anne and Nicholas are trying to sleep and Victor keeps opening the curtains, Anne gets up to close them, and then you hear Victor run across the room and open them again. However the silhouette that is supposed to be Victor opening the curtains appears to be Anne's. You can plainly see her longer hair, and when we see Victor in the end, he has a very short haircut.