I don't think I've ever intentionally ruined endings for anyone. I mean, if they ask, I'll tell them, but I won't just spring it on them. There are several movies I won't watch, because I know how they end. For example, "Grave of the Fireflies." I know enough about the movie from what my friends have said, and even though I absolutely love Miyazaki/Ghibli films, I will not watch this movie. I don't feel like being hit with horrible depression for a week, thanks.
I enjoy pulling little pranks on people, though. After the final Harry Potter book came out and I read it over the weekend, I did this to several people. When someone would say they were still reading the book, I'd say, "Did you get to the part where..." and watch them tense up, their eyes widening and mouths dropping open in panic. Then I'd say "...Voldemort kills Harry's parents?" and they'd instantly relax and laugh with relief.
Sometimes I even make up stuff about books/movies/tv shows to try to entice people to read/watch them, even though I make it obvious that I'm joking around about the content. So in this season of Kyle XY, Amanda and Hillary totally make out, and Kyle saves Seattle from a wave of half-orc zombie ninjas using his super mind-control powers, and Josh starts his own porn company, and Jessi chases Sarah down and they have an epic light saber battle... Heh, I wish. Amanda's mom does get pretty drunk, though.