The Golden Sample: Why Testing Alone Isn't Enough

What's the most common response distributors receive when asking suppliers if their products are safe? "We have tested our product." Yet in spite of this testing, I've heard countless stories from distributors and suppliers alike about how they have had to refuse product during the warehouse receiving process because inspection or random testing revealed that the product failed to meet required safety standards. While this is certainly a problem for the shipment in question, the issue is much broader. Who's to know how many other suppliers have received unsafe blank goods from the same factory as in the above example?