Unintended consequences are outcomes that are not (or not limited to) what the actor intended in a particular situation. Unintended consequences are not just undesired outcomes, it is an unforeseen result of a deliberate action, intention or policy where the outcome is not among the intended goals.
Offer money for something, and people will find ways to supply it, even if the outcome is the opposite of the intended consequence. A nice story about a snake plague goes like this: The government in India long ago tried to prevent the spreading of a snake plague, so they offered a high reward for every snake that would be caught. The result was that the people started to breed snakes in farms in order to make a lot of money.
- Funding fire departments by the number of fire calls made is intended to reward the fire departments that do the most work. However, it may discourage them from fire-prevention activities, which reduce the number of fires.
- Paying architects and engineers according to what is spent on a project can lead to excessively costly projects, because expensive designs are encouraged and rewarded.
- Paying someone contingent on their performance will increase the motivation to perform. However, if they are unable to perform well because the task is hard, because of economic conditions or whatever, you have also given them an incentive to cook the books, as a study reports.
Terror and Blowback
In CIA jargon, "blowback" describes the phenomenon of supporting a foreign regime or terrorist entity, on the principle that your enemy's enemy is your friend, only to have it attack you, often with the weapons and resources you gave it. Examples include:
- US support of Colonel Manuel Noriega
- US support of Saddam Hussein
- US support of the Mujaheddin in Afghanistan, most notably Osama bin Laden, and later the Taliban
- Allied support of Ho Chi Minh and the Vietminh against the Japanese in WWII.
- Royal Empire support of pirates in the 18th century
In the popular modern imagination, pirates of the classical period were rebellious, clever individuals who operated outside the restricting bureaucracy of modern life. They are surrounded by myths and legends. Yet from a modern point of view, the pirates of the 18th century were also terrorists. Piracy after all is just robbery committed at sea, without a commission from a sovereign nation. It is interesting that the golden age of pirates began after the naval wars between England, Spain and France. Many pirates who have looted ships in the name of their king before now did the same for their own sake, because the state pirates were no longer suited for the new politics. The royal empire created it’s own terrorists, just like the USA created it’s own terrorists today. The story roughly was: a kingdom supports pirates to weaken the enemy, the pirates loot the foreign ships successfully, times change and the autonomous parties do something that is not intended by the superpower, for instance they loot ships of the crown, and finally the kingdom turns against the autonomous parties, and the kingdom uses the royal navy to eliminate the pirates.
The general blowback mechanism which results in terror as an unintended consequence includes four steps
- an empire or superpower first delivers weapons and military knowledge to autonomous parties or groups in instable regions
- then the autonomous parties succeed in their conflict, fight or resistance against something
- Times have changed and the autonomous parties do something that is not intended by the superpower
- Finally the superpower turns against the autonomous parties, threatens them or tries to eliminate them
- Wikipedia entry for Unintended consequence
- Museum of unintended consequences 
- Blog post about unintended consequences