Unintended consequences are unforeseen and/or unplanned results of an intent or plan. In most cases, the term is used when referring to negative consequences.
The Great Depression
- The Federal Reserve system had been established to prevent what actually happened. It was set up to avoid a situation in which you would have to close down banks, in which you would have a banking crisis. And yet, under the Federal Reserve system, you had the worst banking crisis in the history of the United States. There's no other example I can think of, of a government measure which produced so clearly the opposite of the results that were intended.