The cobra effect is when an action undertaken to improve things backfires with unintended consequences. It is an unwanted effect of marketing something, such as when (many years ago in India) there was a bounty placed on actual cobras, to solve the problem of "too many cobras running wild." But enterprising people started breeding the cobras in order to collect the bounties. Then the bounties were cancelled and the no-longer-valuable cobras were released, which resulted in many more wild cobras than they started with. Another example is Michelle Obama's school lunch program, implemented to help reduce the wave of obesity in American children. The program only served unwanted foods to students who then wouldn't eat what they were served (even though it was "free"), sending them elsewhere for snacks at lunch, or making them bring normal ("junk") food from home. The result was even more obesity.
The Cobra Effect is not the same as and must be distinguished from COBRA, which is a positive marketing result.