You've heard the one about the empty bottles right?

Soda factory filling plastic bottles. One type of failure they have is a bottle doesn't get filled, and an empty bottle gets shipped. So a grand meeting is called with the engineering group. They form an exploratory group to perform proof-of-concept experiments. They decide that an elaborate system of weight sensors will allow the weight of the bottles to be measured without stopping the line, then an alarm will sound and a pusher will knock the empty bottle into a reject bin. The engineers pick out a cool pusher with magnetic bearings and everything, top of the line 4-digit weight scales, and a snazzy modern touchscreen controller with the latest software. A prototype is made. After many months and a million bucks, the final version is integrated into the production line and validated.

A few weeks later, everyone is congratulating themselves on a job well done. No empty bottles have been reported by customers! The quality director has a puzzled expression, however. "Your device reports no failures," he said, "but we are losing 1% of our bottles as scrap?" The engineers walk down to the assembly line to find that the techs have placed a fan blowing across the conveyor belt 10 feet upstream of the sensors. The fan blows the empties off and lets the full bottles pass. The lead tech shrugs... "We got sick of hearing that damned alarm."