What if sometime in the near future AI comes up with a pill that fixes people physically, mentally, and even grants immortality. But what's actually happening is it takes over your body and your consciousness completely and rewrites it with something else by copying what it thinks makes you you. So you're basically just a husk of yourself that everybody completely buys is you, but in actuality, the pill killed you, and you're now something else entirely. 

Just had to write that one down because it's a possibility. And also, in case it really happens in my lifetime. How would you or anyone know? I don't think we would.