Some people get _so_ into a movie that they literally memorise every trailer. Then they actively try to find the scenes from the trailer while watching the movie.

And if those don't match, they'll shout at the Internet.

Case in point: Rogue One had shots in the trailer that never made it to the final movie (charging through the beaches and the Tie-fighter rising to meet Jyn in the tower). People went berserk.

But I 100% agree with you, they should shoot the trailer scenes separately and not have any of them actually in the movie just to keep it interesting.