There are two twists which should never be twists in your horror fiction. NEVER. I don't like the twist in short stories anyway, because there's not enough room to both develop characters AND set up a twist, but these twists should never occur in your fiction, because these are movie twists and fiction isn't film, and because they suck.
Since Sixth Sense there have been 4 types of American horror movies: the slasher movie, the monster movie, the dead-but-don't-know-it movie, and the split-personality movie. This is hyperbole, but almost not.
I'm going to spoil as few movies as possible, don't worry. I do feel bad for the kids who haven't watched Sixth Sense yet. The beauty of that reveal was an incredibly rare experience. If I had one of those Men in Black mind-wipers, I'd just sit at home and watch Sixth Sense over and over. To head off argument, I'm gonna say that I loved Jacob's Ladder. The trend just seemed to start after Sixth Sense.
It's gotten to the point where, when watching a horror movie, I can see the dead-but-don't-know-it twist as it occurs. I can call with 99% accuracy the moment where the main characters die, and I mean as it happens, not in retrospect. I keep watching because usually the movie has something else to offer, but it's still disappointing to know I'll have to sit through so much that the creators assume will be confusing and freaky when I know exactly what's going on. I can also spot it in fiction. So can everyone else. Don't use it, or at least don't expect the impact of your story to come from it.
The split personality twist is harder to spot. It's usually a variation on slasher. There's the killer, the hunter, and the victim, and the killer can also be one of the other 2 or all 3 of them. It's very reductive of the actual mental disorder, so while it seems to explain all sorts of freaky stuff realistically, it doesn't because it's based on a fiction of a real disorder, meaning it's still explaining freaky stuff with unrealistic fiction. It's a harder twist to spot, but it's unsatisfying because it's stupid.
I have many leather-bound books and a pipe and a deep voice and opinions and I'll tell them to you as fact blah blah blah.