Only partly kidding...
Let's suppose that a thousand, or a million, years ago there was an explosion of some gigantic star (or other celestial object) that was strong enough to destroy Earth and Life As We Know It in this sector of the Universe. But it happened a thousand, or a million, light years away, so the event has not yet come to our attention.
What would be the implications of this (beyond Stack Exchange being wiped out)? Would humanity likely get a year's warning? A hundred years? (I won't ask what Humanity would do with the knowledge, because we all know it would make the stupidest decisions possible.)