People were pretty good about keeping the secret of The Sixth Sense, The Crying Game, and even Fight Club. But let's not put it past the movie companies themselves to keep the cat in the bag about their own films.
Last week I caught a commercial for Hancock - a movie I wasn't necessarily thrilled about but even when I was discussing it, I refused to give away the "twist" of the film, no matter how lame it is. Anyway, so there's the commercial and--BAM--there's the "twist" right there in the middle of this 30-second spot... big as life.
It's fairly obvious that they still have no idea how to treat this movie, as evidenced by this spot and by the recent news of some various cuts of this film coming out.