Citing Wikipedia : (this concept) “… originated in an anecdote set at the time of British rule of colonial India. The British government was concerned about the number of venomous cobra snakes in Delhi. The government, therefore, offered a bounty for every dead cobra. Initially, this was a successful strategy as large numbers of snakes were killed for the reward.
Eventually, however, enterprising individuals began breeding cobras for income. When the government became aware of this, the reward program was scrapped, causing the cobra breeders to set the now-worthless snakes free”
When designing incentives, thinking like a thief can be helpful sometimes. And even when everything is carefully planned, one must keep an eye on the results, as there may not be what is expected.