Revealing mistake: At the beginning of Angela's bathroom death scene she is looking at herself in the mirror. You not only see her reflection in the mirror, but also in the shiny metal strip holding the mirror at the bottom which is to be expected. When she turns away and her 'demon' reflection is watching her, you still see the image in the metal strip. As the 'demon' is in the mirror you should no longer see anything reflected in the metal. This does seem to show how these moments were filmed, with the real actors reflections being recorded.
Other mistake: In the scene where the mirror cuts his hand, Ben touches the mirror with his left hand. In the very next shot he is holding the flashlight with his left, pointing on his wounded right hand. Right after that he switches hands again. (00:23:35)
Revealing mistake: In the scene at the morgue, after Amy Smart's death, Keith Sutherland is holding her hand before putting it under the sheet. Her hand should be cold, but when it's moved a hand print is left on the steel slab due to condensation from her warm and very much alive hand. (01:36:45 - 01:37:55)
Other mistake: In the scene where the son is woken up by the "screaming woman", when the mother rushes out of her room you can see the sun in the window behind her, despite the the fact the scene takes place at night.
Continuity mistake: Just after fake Amy in the mirror starts to cut her daughters neck slightly with the scissors, the real Amy having escaped the bathtub grabs her daughter and puts her in the corner of the room. Soaking, she removes her top exposing her right breast and nipple - next shot she has a soaked singlet on which wasn't there a second ago.