Jessica: Rowena's changing people's fates. She's killing them before their time, and when a reaper shows...
Sam: She torches them, too.
Jessica: Yes.
Dean: Why?
Jessica: You'll have to ask her when you stop her. Because if she keeps this up she's going to throw off fate. The whole greater machinery of death.
Sam: That means?
Jessica: Have you ever heard of the Butterfly Effect?
Dean: Ashton's second best movie.