This happens to me all the time, too. I remember when I went to see 'The Matrix'. There was this wonderful bit of foreshadowing (OK, it wasn't wonderful, but it was passable) where the following exchange takes place:

Neo: You mean I can dodge bullets?

Morpheus: When you're ready, you won't have to.

One of the trailers played these lines, and then SHOWED THE FRIGGIN SCENE THEY REFERRED TO DESPITE THE FACT THAT IT WAS IN THE LAST TEN MINUTES OF THE MOVIE.

And this was from a movie that was playing up the mystery factor big time. When I see something like that, I just want to find the advertisers and tear their eyeballs out with a rusty salad fork.

This isn't just a problem with movies, though. I know I've had several books spoiled for me (in either major or minor ways) just by reading the jacket blurb. In fact, it's gotten to a point where I don't read them unless I've never heard of the book/author before, since usually I'm buying books on the recommendation of a friend anyway.

I think the only way to avoid any of this is to hide your head under a pillow every time you see an advertisement for any sort of entertainment anywhere. In fact, I think I'm going to do that from now on.