I seriously once read a thread about Serenity, that had no spoiler tags on one of the post that read
"I cried when Snape killed River."
At first I was furious that River's death was spoiled for me.... then I realized that Snape definitely wasn't apart of Firefly, and did a double-take. Got a good laugh out of it.
But from now on, any time a highly anticipated movie comes out, you can give a fake spoiler that Snape killed someone, and everyone will get what you mean whether they are a fan of HP or not.