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.
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)