You've heard those medieval horror stories, whether true or fabricated: A body is prepared for burial, only for the person to jolt back to life while lying in a casket at their own wake. (Fun fact: Wakes did in fact begin as a sort of pause before the burial just to really make sure the body was lifeless.) Worse yet are the stories of people who were buried alive. How could this happen? Well, turns out deciding when someone is dead is kind of confusing.
According to BBC, "biologically speaking, there has never been a single moment of death; each passing is really a series of mini-deaths, with different tissues dropping off at different rates." As Robert Veatch from the Kennedy Institute of Ethics told them: "Choosing a definition of death is essentially a religious or philosophical question."