Law Number XLV: One should expect that the expected can be prevented, but the unexpected should have been expected.
-Norman Ralph Augustine, Augustine's Laws
There are tons of aphorisms surrounding this topic: "expect the unexpected" is a particularly common one. Or if you've come in contact with any of NASA's safety discussions (or heard it from Donald Rumsfeld), you know about the "unknown unknowns." What this all boils down to is that risk presents itself to us in the form of what we know versus what we don't, what we expect and what we don't.