In 2003, then U.S. Secretary of Defense Donald Rumsfeld won Britain's Plain English Campaign 2003 Golden Bull Award for this statement: "Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns, there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns - the ones we don't know we don't know."

What possibility did Secretary Rumsfeld omit?