Originally Posted by cestmoi123

If memory serves, one of the aircraft engine manufacturers had a similar problem with people inspecting jet engine turbine blades. Defects were so rare that, one the very rare case somebody did come across one, they were likely to miss it, since they might go through thousands of blades before finding a defect. So, they started intentionally introducing a very small number of defects before the blades were inspected, to keep the inspectors mentally engaged. These weren't lazy or incompetent people - they were just human.