As in, you see a movie trailer, and based only on that trailer you make up the whole movie in your mind, and it ends up being different than the actual movie. Was your version better or worse?
I’ll go first: Men In Black 1 had a somewhat misleading trailer, where they’re about to shoot down the flying saucer at the end, and they say to each other “Do you have any idea what you’re doing?” “Not a clue.” And they shoot. So in my mind it was two guys from the FBI who had to suddenly deal with the existence of aliens and learn to fight them on the fly, learning and making it up as they went along all the while learning to work with each other.
I often get dreams about watching the movie and being disappointed.