can be a disaster, surprise
It means that one never knows what will happen in any situation. It is a useless warning because one can't actually expect the unexpected. Remember 911.
But did anyone believe it would happen to them. Not if they went in the buildings. To live in fear of the unexpected is not normal especially from fiction or previous events.
It's a STUPID ex<x>pression, much like "IT IS WHAT IT IS"
it means anticipate not only the good that will happen but the bad as well...