Ever get annoyed when you watch a promo for a show or movie and they use all the best lines and scenes? Or they reveal something that happens way at the end of the plot? Books have a bad habit of doing the latter one. In trying to make the blurb on the cover grab a reader's interest, they give too much away. This isn't a problem when you buy the book, because you have no idea when the events that are being described will occur. Sometimes this tactic is used because very little happens until the events described on the back of the book (a bad thing, very bad). Sometimes it's the product of a bad copywriter. I know of a couple cases where the authors try to get the word out to their fans and readers to not read the back cover blurb or the teaser.
I have no idea which of the two (or some other reason that I can't drum up at the moment) led to the back cover copy on a recent book I read, but the event described on the back of the book that leads you to believe the book will be about that particular thing doesn't occur until about page 420 of 480. I shit you not. I'd provide specifics, but if I told you the book or the event, I'd end up spoiling the book for you one way or another.
I've never seen such a terrible back cover copy, and I really hope that the person who wrote it got fired for it. There were plenty of other ways to make this book exciting to someone just browsing the SF aisles, and doing things this way only served to misrepresent the book and make for a disjointed read. I feel so bad for this author, and I hope I never run into this problem when I get published.