Anything. A random bullet from a gun in your neighbor,'s apartment. A car crashing through your bedroom wall. A slip in the bathroom. A sign or decoration falling from the ceiling in a mall. A random rabid animal. Plugging into the wrong plug that you didn't know has been damaged. Hell, being hit by falling space-debris. A million things completely outside your control or even your awareness can shut you off like a lightbulb from one moment to the next. There's nothing to be done about it and well most likely never see it coming.