…when they spoil too much of the story or they mislead you to what the movie is about?
Take for example Prime: The trailer promised a funny comedy about a woman that has a relationship with the son of her psicanalist and no one of them does know that. The movie was a sad story, with few LOL moments, about a woman that tried to accomplish a relationship with a man younger than her and didn't succed in it. Even my girlfriend was disappointed.