Noone can tell when I spoil movies, books, and such. My friends and I sat down to watch something once. I think it was supposed to be a romantic comedy or something. But I just turn to the group who hasn't seen it yet and said:
"Just letting you know... That guys dies."
or
"Everyone dies by the end."
My friends have stopped taking me serious when we were watching something and I blurt out "Snape kills Trinity with Rosebud." (stolen from XKCD) while we were watching Plan 9 from Outer Space.
Aside from that, my mom was trying to get me to finish Harry Potter 7 before the movie came out. And while discussing that I was only on Chapter 8(?), before anything spoilerific happened, she says: So you don't know --omitted-- dies? and she also gave the ending away before I got any farther, and tried to shrug it off saying: "Nah, that couldn't happen."
I started reading The Hunger Games, and she saw the movie before buying the books. While I was reading she asked where I was, and I told her... "Is that before --omitted--- happens?" She spoiled half the book for me too and I was only like 3 chapters in.