Deliberate mistake: When Eddie and "Jennifer" are walking around the house alone, she is trying to tell Eddie that she's really Sara. But by the time she is explaining that, Eddie has disappeared. She doesn't find him until later. But from the moment she finds him (and he is talking to her) he calls her Sara, despite never actually hearing her say that. Also, after they all get back together, a few of the other people start calling her Sara as well, but no one actually heard that either.
Deliberate mistake: In the scene about the Terror Incognita roller-coaster, when the rails break, they have ragged ends at the gap; after the first roller-coaster train derails, the rails move together again to close the gap and allow the second roller-coaster train to pass safely; but as you see the rails move together again, they now have neatly cut ends at the gap, not ragged ends. A deliberate error to mislead the audience, but an error nonetheless. (00:10:52)
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.