1) Titanic. Turns out at the end that the old lady telling the story was Jack, not Rose--Rose dies in the sinking, Jack survives, grows old and has a sex change. He assumes Rose's identity to steal her fambly fortune.
2) The Notebook. Turns out the old guy who reads to the old woman was not in fact her husband, just a creepy old derelict taking advantage of a woman with Alzheimers in order to sleep with her.
3) Blair Witch Project. Turns out the three film students were wandering around in Manhattan's Central Park the whole time, just going in really small circles.
4) Thelma and Louise. Turns out the cops chasing them were just trying to give them a fine for littering.
5) The Matrix. Turns out Neo and his pals are imagining the whole thing on some powerful drug, and just went around slaughtering a lot of innocent people.
6) The Excorcist. Turns out the kid was faking it the whole time just to get out of school.
7) The Towering Inferno. The skyscraper was never actually on fire, it was just some hillbilly couple on the 60th floor trying to fry chicken in a waffle iron and making way too much smoke.
8) Footloose. Ren, the city boy who brings fancy dance moves and rebellion to a repressed hick town, turns out to have been the drunk driver who got all the other kids killed years ago when they were driving back from a dance, causing the dancing ban in the first place. He uses dance now to forget his crimes.
9) Psycho. Turns out the killer is not Norman Bates or his mother, but rather--a bunch of birds all gathered under a housedress and wig!
10) Lethal Weapon. Riggs and Murtaugh aren't just a mismatched pair of cops, but rather an anti-Semitic religious nut and a Castro-sympathizing leftist moron.
11) King Kong. Turns out the ape is not a giant ape at all, just a normal sized gorilla that gets filmed in close-up. All the New Yorkers are just overdramatic about apes running wild.
12) Road House. Turns out that pain does, in fact, hurt.