“The ideas of economists and political philosophers, both when they are right and when they are wrong, are more powerful than is commonly understood. Indeed the world is ruled by little else. Practical men, who believe themselves to be quite exempt from any intellectual influence, are usually the slaves of some defunct economist.” - John Maynard Keynes
Keynes was half right. It's not so much that practical men are the slaves of some defunct economist or philosopher's ideas - it's that they are the slaves to the only-half understood idea of some dead economist. Adam Smith, who also wrote about how tradesmen constantly conspired against the public good and who wrote an entire book on morals, would have been horrified by the way his passage on the invisible hand has been used to justify never interfering in markets, for example. And Marx, perhaps most succinctly, once said "I am not a Marxist".
Every era is subject to a number of bad ideas, born of badly understood philosophy (in the larger sense, where all the sciences are but offspring of the first discipline, that of the philosopher). In our era one of the worst is what has come to be known as "free market fundamentalism".