Colonial Hanoi paid a bounty per rat tail and got more rats
The metric was supposed to measure a shrinking problem. It measured something else entirely — and the city's rat population answered accordingly.
In 1902, French colonial administrators in Hanoi tried to solve a plague-linked rat infestation the way any manager would today: pay per result. Bring in a rat tail, collect a bounty. Over a few months, more than four thousand tails a day were turned in. By the numbers, the campaign was a runaway success.
Except the rats weren't disappearing. Officials sent out to verify started finding tailless rats running around the sewers — trappers were cutting the tail for the bounty and releasing the animal to keep breeding, because a live rat produces more future rats to harvest than a dead one does. Some residents were reportedly farming rats outright, breeding stock they could tail on a schedule.
Economists later gave this pattern a name — the cobra effect, after a similar bounty scheme in colonial India that allegedly produced backyard cobra farms — but Hanoi is the best-documented case. The incentive wasn't badly designed by accident. It was well designed for exactly the behavior it produced: maximize tails delivered. Nobody had asked for fewer rats. They'd asked for more tails, and gotten precisely that.
Every metric you set is a bounty on tails. The gap between what you're actually measuring and what you think you're measuring is where the real behavior lives — usually multiplying, out of sight, in the sewer you didn't check.