As an instance of the nonsense or spurious correlation that is a real statistical fact, someone has gleefully pointed to this: There is a close relationship between the salaries of Presbyterian ministers in Massachusetts and the price of rum in Havana.
Which is the cause and which the effect? In other words, are the ministers benefiting from the rum trade or supporting it? All right. That’s so farfetched that it is ridiculous at a glance. But watch out for other applications of post hoc logic that differ from this one only in being more subtle. In the case of the ministers and the rum it is easy to see that both figures are growing because of the influence of a third factor: the historic and world-wide rise in the price level of practically everything.
http://www.burns.com/wcbspurcorl.htm