“Expect the unexpected” seems like a contradictory and almost useless phrase. Unexpected events are inherently unknowable, so how can we prepare for them? A better interpretation might be to prepare for everything we can know about, so our efforts can be focused to overcome the unknown.
Inevitably there will be a surprise or emergency for which no one is prepared. If there are too many loose ends and unfinished details, the unexpected becomes impossible to deal with. There is simply no energy or focus left to channel towards that problem.
But if you have diligently settled all other tasks, made sure that every possible known angle is covered, you can stand ready and prepared for whatever comes your way. So don’t just “expect the unexpected,” prepare for the unknown by accomplishing as much as possible before it hits.