In computer science there is a well known theory about testing and the intractability of obtaining 100% certain proof of bug free code. To summarize it "Testing can never prove that software is free of all bugs"
Explaining that it is impossible to do to the Layman and beings of lower standing such as high paid wild eyed, foaming members of upper management in tailored suits who think that they require a megaphone to be heard. Can be an entertaining experience..
Come to think of that have you ever had the experience of a ticked of manager screaming at you and all you can think about is how much he looks like 2 year old child having a tantrum in a store while the parent quietly ignores them and continues to shop... So you sit like the adult, waiting, counting the mans heart rate by the huge pulsing vain in his forehead.. Wondering the whole time if your going to need the AED from down the hall or not....
By now his rampage is pushing that last of the oxygen out of his system and he is starting to turn that deep shade of purple and you know that he has to take a breath soon so you get ready and as he sucks in that life giving air you shoot off "Oh.. i sent you an email about that 2 weeks back boss.. you did read it right?"
And then it suddenly dawns on him that the unread mails from you between his playboy subscription renewal and the face-book update messages might have been important... As the oxygen rushes back into his system his face turns a sudden bright red your are left eternally wondering if he actually blushed or if it was just his flesh re-oxygenating...
Ahh give the man a break. The truth is he doesn't understand crud...he knows it... we know it.
His high paid job is all about making promises about stuff that he doesnt fully understand, talking about the work that you are doing and listening to you dribble on about the big O complexity of the search functions... And try as he might to listen to you all he can think about is that hot blond from last weekend... So now he is pissed because he thinks you deliberately lead him to the cliff edge that he just fell off and he is trying to swim back up through the air like wile-e coyte...
The reality of the situation is that this is neither good for you or him. So re-educating the boss is the best course.. the way i seem to have had the most success explaining the inability of testing to find all bug is by a coin flip game.
Consider software to be a set of coins (2 or more). The running of software is a coin toss. And bugs are when all the coins show heads. SO testing the idea of tossing the coins until you get all heads and find a bug.
Now with this in mind, walk him through a "trivial piece of software" ie a game with 2 coins. Explain to himvthat the chance of finding a bug is 1/4 so it will take about 4 tries to find it. Yes i know "4 tries" is not really accurate but kept it simple alot of people don't get or care about the mathematical background to the average.
Then expand "a 3 coin piece of software" which has a 1/8th of a chance to find a bug and therefore it will take 8 tries to find a bug. Make certian to pound home that fact that the size of the program causes an exponential growth in the cost and time of testing.
Then you hit him with that fact that your program is really something that is thousands of coins big and it would be somewhat far out of his budget to have it all tested so that it is truly 100% bug free. And there you have the seed of knowledge.. at least until the boss drinks his next set of neurons into oblivion..
Later on you you can take this metaphor a bit further by explaining that
* directed testing is like weighting the coins to come out to a point that you think has bugs
* coverage and expensive coverage tools all are about recording outcomes of the coin toss so that we can tell what % of all the possible results we have seen and take a guess at when to really stop testing.
* etc etc...