After Helen first fetches the ether from the bathroom cabinet, she leaves the bathroom without closing the cabinet door. No-one goes back to do anything with it, and seconds later the cabinet door has closed itself.
When Helen lights the candle in a candle holder in the kitchen, she holds it on the bottom with her left hand, blocking the breeze with her right hand. The next shot is of her leaving the kitchen, still blocking with her right hand but now hold the candle stick near the top of holder instead of the bottom. She starts down the spiral staircase and as she rounds the first turn, she's once again holding the candle holder on the bottom.
In the long conversation between the old lady and her son by the bed (half an hour into the film) he is seen from one angel to clutch her arm, and from another angel her both arms are free - alternately throughout the scene.
SPOILER:Mrs. Warren is presented as bedridden yet, when Helen is searching for the gun she checks the entire room - though this may suggest that she already knows her employer is not such an invalid as she makes out.
When Helen first walks up the main staircase, she pauses to look at herself in the mirror. In the next shot as the camera slowly rolls backwards, she is still looking in the "mirror" but there is no reflection of herself (just a painting designed to look like mirror).