Major source of mistakes in economics. Most of modern academic economic research is pursued in the following fashion:
/1/ Download some dataset. Anything you can get. The larger the better. Think “Big Data”, the most recent fad. Everybody's crazy about it.
/2/ Torture the data by a statistical software package until they sing. Find a correlation.
/3/ Bingo. Your paper is about to be finished. Just find or invent some theory, no matter whether half-baked or completely raw. And you're done.
This is the easiest way to get your work published and earn some degree. No matter if the correlation is bogus, fallacious or spurious. No matter your work is worthless. Worst thing, which may happen, is that your paper would be followed by some similarly half-baked academics or central bankers.
ODD FACT: Flies are no tourist attraction
The fake Nobel Prize winner Ragnar Frisch used to warn his students before spurious correlations by giving the following horrifying illustration. It can be observed that there is a high positive intercorrelation between the number of flies on the western coast of Norway and the number of tourists visiting that region. From this observation, it is probably not a very good idea to try to promote tourism by breeding more flies. However, the phenomenon of spurious correlation has a more intricate form, which is often much harder to discover. (Excerpted from the Nobel speech by Trygve Haavelmo.)
The most common (and the most harmful) example of spurious correlation is the so-called deflation spiral; cause of wrong monetary policies pursued by many central banks.