This is ridiculously fascinating, and terrifying beyond belief!
A man named Graham (full name omitted for his anonymity) has something called Cotard's syndrome, which is extremely rare. People who suffer from this believe that they, or parts of their body, no longer exist! Graham's surfaced after a botched attempt to kill himself, after getting into a tub with an appliance.
When he woke up, Graham found that he was dead — regardless of the fact that he's clearly not, and that doctors tell him so on a regular basis!
Here's what Graham said about it: