Revealing mistake: In the opening scene of the film we see the patients of the institute smash through a cubicle where a guy is, watch closely. The glass breaks away very very easily, obviously some kind of special easy-to-break glass used for this scene. (00:04:30)
Revealing mistake: When the house has been locked down at the beginning of the film no moonlight could come in, however when the guests are being shown the coffin with the guns in there is a shaft of bright moonlight shining on to it.
Answer: The ghost that kills Melissa is the patient Vannacutt was dissecting. When Melissa initially sees them through her camera, what she's really seeing is the ghosts re-living their last moments.