Heh, I have quite a few pet peeves, like affect/effect and saying there's instead of there're when referring to plurals. But I've spent some time making sure my students know that there is no such thing as 100% certainty for things existing in the universe. But then, you can say "I'm 100% certain that one does not equal two" and that's true, so it's more about being conscientious of the language you use to describe reality.