25. April Fool's Day (1986)
Spoiler Alert! It's all been a big April Fool's Day joke. No one is actually dead. (Apparently, the pranksters are very good at special effects and makeup.) It's a nice idea for a twist in general, but a slasher with no one dying is pretty lame.
24. When a Stranger Calls (1979)
Spoiler Alert! After following the killer around for an hour or so, we suddenly switch back to the babysitter from the opening scene, now grown up with kids. She receives a call from the killer reminiscent of the calls from seven years ago, and although her kids are OK, can you guess where the killer is? That's right, in the house. Fool me once, shame on you; fool me twice, I'm dead. Not only is he inside the house, though, but he's IN THE BED!
23. Identity (2003)
Spoiler Alert! The people in the motel are all personalities in the head of a killer sentenced to die. He's been given a special treatment designed to "kill" his alternate personalities, leaving a kinder, gentler killer.
22. The Descent (original ending) (2005)
Spoiler Alert! The original European ending was a much more depressing -- though probably better -- affair than the American ending. In the original version, the last surviving spelunker finally escapes the cave and drives away, only realize that her escape is a hallucination, and she's still trapped in the darkness. The end.
21. Fallen (1998)
Spoiler Alert! We're set up in the opening scene by Denzel's voiceover, saying that he almost died during the ordeal. What we don't realize is that Denzel is possessed by the killer while he's speaking, so in fact it's the killer who almost dies, while Denzel, sadly, bites the bullet.
20. Carnival of Souls (1962)
Spoiler Alert! This is one of the first films to use the plot twist of the protagonist being dead all along, which now has been beaten to death. Enough, people!
19. The Others (2001)
Spoiler Alert! The house is indeed haunted...by them! The woman and her kids have been dead all along and have been bothering honest living people trying to make a living. The ending might've had more of an impact had it not come on the heels of The Sixth Sense.
18. Scream (1996)
Spoiler Alert! The killer is...two killers! Neve Campbell's boyfriend and his friend team up to play "Stab the Student Body."
17. The Cabinet of Dr. Caligari (1920)
Spoiler Alert! It turns out that the narrator is the real madman, and he's telling his tale in an insane asylum. Caligari is in fact his doctor in the asylum.
16. The Mist (2007)
Spoiler Alert! Five characters drive a truck out of town, only to see that the creature-filled mist goes on forever. They run out of gas and are so despondent that they decide to commit suicide rather than be killed by the monsters. Since they have only four bullets, David (Thomas Jane) shoots the other four people -- including his son -- and steps out of the truck to let the creatures kill him...except the Army shows up, saving the day. Whoops! While the ending doesn't fully work -- partly due to Jane's acting -- it takes guts to even attempt.