Continuity mistake: When Sara is first put in the attic as a servant, she has a long candle with no wax dripping. She puts the candle in a holder and it is short with a lot of wax on it, then the candle is long again.
Continuity mistake: When Sara first goes into the attic, she starts to draw a circle around her with a piece of chalk. When she starts the circle, she starts drawing next to the candle. When she completes the circle, the candle is now inside the circle. If you look at the lines on the floor when she finishes, you can see that she did not go around the candle.
Continuity mistake: At one point the Head Mistress gets mad at Sarah in the attic and when she leaves a candelabra, the height of the candles change from shot to shot.