The Cobra Effect - or more generalized know as the Perverse Incentive - describes effects contrary to the intention, brought about by people that are hacking the system to gain advantage.
The term originated from an anecdote about the British in Deli, who wanted to get rid of an increasing cobras population. Hence they paid everyone that brought in a dead cobra. Hence people started breeding cobras, to gain more money. Once know, the British stopped paying out money for cobras and the result was more cobras in the city.