A soap factory had a problem.

[retired]

They sometimes shipped empty boxes without the bar inside. This challenged their perceived quality with the buyers and distributors. Understanding how important these relationships were, the CEO of the company assembled his top people. They decided to hire an external engineering company to solve their empty boxes problem. Six months, and $8 million later, they had a fantastic solution – on time, on budget, and high quality. Everyone in the project was pleased.
They solved the problem by using a high-tech precision scale that would sound a bell and flash lights whenever a soap box weighed less than it should. The line would stop, someone would walk over, remove the defective box, and then press another button to re-start the line. As a result of the new package monitoring process, no empty boxes were being shipped out of the factory.
A while later, the CEO decides to look at the first week report. Since the scales were put in place, no empty boxes had been shipped out of the factory. Each day about a dozen defective boxes were being removed, which was consistent with the projections. There were almost zero customer complaints and they were gaining market share. The CEO felt the $8 million was well spent.
However, the number of defective boxes picked up by the scales dropped to zero after three weeks. He filed a bug against it and after some investigation, the engineers came back saying the report was actually correct. The scales really weren't picking up any defects because all boxes that got to that point in the conveyor belt were good.
Puzzled, the CEO traveled down to the factory, viewed the part of the line where the precision scale was installed, and observed just ahead of the new $8 million dollar solution sat a $20 desk fan blowing the empty boxes off the belt and into a bin. He asked the line supervisor what that was about.
"Oh, that," the supervisor replied, "Bert, the kid from maintenance, put it there because he was tired of walking over, removing the box and re-starting the line every time the bell rang."