What is the one plot twist you will never understand?S

No matter how hard I try, there are certain plot twists I will just never understand. Sometimes it's because they come out of left field and leave me wondering what the hell the writers were thinking. Other times, it's because the twist is just too freakin complicated for ordinary humans to grasp. What is the one plot twist that broke your brain?

Image from eXistenZ

You can choose from books, movies, comics, TV — whatever. And yes, there will be spoilers because we're talking about plot twists! Please include a link, picture, or clip with your post so we can all appreciate the mind-bendingness for ourselves.