From PHP Planet today, there's a note on Marcus White's weblog stating that: you cannot prove anything by testing. No matter how hard you try, there could always be something wrong, or some combination of conditions or some external event that you haven't thought of. You never know absolutely for sure that your project is working correctly and you never will.

He starts off talking about the nature of proof and how it is so hard to believe. His example? Swans - You live in a village and all you have seen today are white swans, from which you conclude all swans are white. A little rash perhaps, but logical. Now this isn't yet a convincing theory, so we wander down to the village pond and look for swans. We are not looking for white swans though, we are looking for non-white ones.

Just remember, there's no way to get it 100% perfect. There are a lot of people out there that might try to, but ultimately, you just have to work wround the issues...