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.
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.