Much like the sun, movie trailers are not meant to be looked at directly if you want to go into a movie blissfully unaware of its twists and turns. However, making it to the theater without having a movie spoiled by trailers, actors, or even merchandise is harder than you'd think.
To get as many people into a theater as possible, trailers for a film will often show all of the coolest parts of a movie, even if the coolest parts are meant to be a surprise. Movies that spoil themselves this way can ruin the pure joy that comes with the total shock of the appearance of a beloved character, or a plot point that's unexpected. Toys that spoil movies are much more disappointing. They're often released long before the film and include extras that give away major plot points.
Obvious warning, but just in case: In you haven't seen some of these films, spoilers ahead!