I stumbled on this while trying to catch up on posts at Overcoming Bias. If you feel 100% sure of anything (not 99.9999% sure) it means that you are never allowed to change your mind, ever. Not if the facts change, not if you get new facts, not if the laws of physics change and not if God himself says you were wrong. So long as there is a non-zero chance on any of these things happening (and the odds that we're all just hallucinating are surely not zero), there's a chance that we're wrong.
Saying, "Given the evidence as it is, I'm 100% sure, but I'll change my mind if new evidence comes along." is no better than saying, "60% of the time, it works every time!"
And being 99.9999% sure is really sure. It means that if you collected a million things you're that sure of, only one of them would be wrong. Maybe that'd work for you, but if you randomly chose ten thousand people and asked them each to list a hundred facts that they're 99.9999% sure of, you think only one statement would turn out to be wrong? Keep in mind that about half of first year physics students don't know that the moon goes round the earth (O please God let me be right about that).