I think you need to warn about spoilers in this thread! But, for me, the biggest surprise in a recent movie was Leslie Burke’s death in Bridge to Terabithia: I mean, this is an American children’s movie, and sympathetic main characters don’t die! (Of course, I didn’t read the book until after seeing the movie).
The Sixth Sense. You know the twist by now, I assume. I worked at a video store at the time, and EVERY SINGLE MOVIE GOT SPOILED FOR ME by customers (Se7en, Crying Game, From Dusk…, Usual Suspects, etc.) except that one. That was the only “big secret” movie I actually got to see without even knowing that there *was *a secret. It. Was. Awesome.
I can’t help but wonder if I would have liked all those others better if I hadn’t known their secrets going in…
Maybe you people can help me think of another situation that I had meant to include in the OP. It’s been in several movies, some more effectively than others, where whatever monster, creature, crazed homicidal maniac, deadly virus, or other entity that usually gets killed off, incinerated, blown to bits, dropped in an acid bath, crushed under a steam roller or otherwise dutifully eliminated by movie’s end, is shown to be continuing as a menace. This device normally flashes the SEQUEL SOON sign for all to see, but in some of the grizzlier movies the end is just left that way to keep you from sleeping well for a while.
All of Fight Club took me by surprise, since it had been promoted as if it were nothing more than a macho movie about tough guys beating each other up. But when we learned Tyler Durden’s true identity, I was so taken aback that I accidentally snorked a piece of popcorn into my windpipe. That movie nearly killed me. But I had to watch it again anyway.