A toothpaste manufacturer was having trouble because empty toothpaste boxes were being shipped to stores.  They hired a consulting firm to find a solution; the engineers developed a very sensitive scale that could tell the difference between a full box and an empty one.  When the scale beeped, the line would halt until the empty box was removed.

The plant manager tracked the results.  For the first week there were 8-10 halts per day, but then they stopped altogether.  Worried that the scale was no longer working, the boss checked things out and found a large fan had been placed near the end of the line.  “What’s this?” he asked.  “The janitor was tired of restarting the line all day, so he set up the fan to blow the empty boxes off the belt.”

“Oh.”