Also we can take this idea even further: what if only 1% of the entries in the guestbook don’t show? Does the code work? What if it’s so badly written it take someone with a really good experience to be able to prove that bug exists at all?

How do you go about proving this? In the real world people are going to dismiss it. If you are playing against a politician you will be told that you simply have a bad memory thinking that some things exist that really don’t.