http://blog.pmean.com/least-important-variable/

The statistician reviewed each variable in order starting with the most important variable. It was rather dull, of course, until the statistician got to the bottom of the list. He proclaimed “and the least important variable is the amount of water in the raw material.”

At this point the engineers in the room burst into laughter. It turns out that water was the most important variable. If you had even a small amount of water in the raw material, the entire production process would explode. The engineers spent a huge amount of effort to keep the water down to a level that was barely measurable.