The Onion's AV Club lists 15 movies that start off going in one direction, then veer wildly into another, taking the films into territories you never expected. Spoilers throughout!
'Psycho' at first plays like the story of a woman of limited means who makes a bad decision, then tensely lives on the outside of the law. On the run after absconding with her boss' money, she slowly turns into a nervous wreck. Then she checks into a motel, meets a nice (though slightly nervous) young man, and comes to a decision: She will right her wrongs and return the money. But first, a nice shower…
From the Upcoming Queue, submitted by mrbabyman.
Epic fail.
Fight Club? Great movie, great twist. But the tone of the movie doesn't really change. We've known all along that the main character is goofy.