To follow up on Gene's recent post...When I was in college some guy was arguing that Hume's critique of induction was stupid. He (in a manner similar to Gene's emailer) kept picking up a pen and dropping it onto his desk, saying, "We're absolutely sure that when I let go of this pen, it's going to hit the desk. I can do that as many times as we want, and it's always going to happen." And to really prove his point, he repeated his experiment a few times.
Now I didn't have the courage to actually do this--I am much bolder on the Internet than in person--but it occurred to me that it would be hilarious if I had suddenly reached out my hand and caught his pen after one of his drops. Then, of course, it wouldn't have hit the desk, even though we all "knew" it had to with perfect certainty.
More generally, there could have been a large electromagnet that someone turned on, or a strong wind, etc. etc. to falsify his prediction. Of course, this wouldn't have fazed the budding philosopher (yes, he was a philosophy major); what he meant was, the pen will hit the desk, unless something prevents it from doing so.
Well, yeah, I don't think Hume would've argued with that. He also would've endorsed the following proposition: Nature always obeys the true laws of physics.
The problem is, though, that the true laws of physics might not be the laws in our textbooks. And even if they were, we could never know this for sure.