expect the unexpected
Be prepared to face unanticipated events or situations. Climbing a mountain has many dangers, so no matter how much you prepare, expect the unexpected.
unexpected always happens
Prov. The things you do not foresee will happen to you; when you plan, you cannot think of everything that might happen. It took us an hour to drive to the restaurant, and when we got there, it was closed. I would never have expected a restaurant to be closed on a Friday night, but the unexpected always happens. Especially to me.