I had no idea that half the movie would take place after Joy and Jack escaped - hell, I didn't even know Joan Allen and William H. Macy were going to be in the movie.
This was me. I had seen the trailer several times, but it managed to not give away those elements, or if it did, I managed to forget them by the time I saw the movie. (If I recall correctly, there was a scene in the trailer of the kid in the back of the truck, but no depiction of his life beyond that moment.)