Did you know that the actor who played Darth Vader David Prowse (James Earl Jones only did the voice) actually gave away the plot of The Empire Strikes Back before the movie even hit the theaters? He told a fan magazine that Darth Vader was Luke Skywalker's father.
Sometimes movies spoil the endings by just their trailers. Here are some examples!
SPEED shows the surprise ending on the bus exploding at the end with the passengers watching.
Free Willy makes it back to the ocean in this trailer promoting the film (giving the film's ending away)