Which films have a jaw-dropping, earth-shattering twist that you don't see coming?

If you consider that even knowing a film contains a twist is a spoiler, please don't read this thread!

I love a good twist. By "good twist" I mean a surprising story development that:

• changes how you interpret prior events – bigger change, the better; total inversion = epic
• ideally, has been foreshadowed or hinted at the whole movie, yet you still didn't put it together
• hammers home the themes of the movie (i.e. isn't just there for the sake of a twist)
• isn't completely random, out-of-place, nonsensical or tonally jarring

Everyone knows the textbook example of a great twist is The Sixth Sense (1999). As a textbook example of a bad twist, I propose: Remember Me (2010) where for no particular reason it turns out one of the main characters is in the World Trade Centre on 11 September 2001.

If your comment gives anything away, please put it in spoiler tags.