An attempted solution to a problem actually makes the problem worse.
Named for an anecdote in India under British rule to reduce dangerous cobras where villagers were paid a bounty for dead cobras, and people began 'farming' the snakes to collect more bounty. The term was later popularized a term popularized by the German economist Horst Siebert (source).
Based on a real set of events, the Cobra Effect reminds us to always consider incentives, and to think carefully about whether or not we are promoting a system to be 'gamed'. When it comes to dieting, for example, many people disproportionately focus on losing weight as opposed to building healthier eating habits and lifestyles, and some so much so that they persuade themselves to make unhealthy choices that prioritize weight over health.