Like planes just vanishing off of radar. We think that doesn't happen these days - but it happened to MH370. (Most likely the plane simply went down, but these kinds of disappearances have all but ended now we have better tracking). It send no distress signal. The pilots didn't say anything - but the plane did start to turn off of its flight path. I have to wonder if something in the flight electronics crashed.
We think we have removed a lot of the mystery from the world - and we have - but things like this can still happen. Can and do. Not that it makes me afraid to fly - planes don't go down very often, after all. But it does remind me that we can't know everything. Even on Earth.