In 1902, French colonial authorities in Hanoi paid Vietnamese workers a bounty per rat tail killed in the city's sewers to halt bubonic plague. Workers responded by severing tails and releasing live rats to breed, and some began farming rats to collect more bounties, so the population rose rather than fell.
When a reward is attached to an easily faked signal of success (a rat tail) rather than the actual goal (fewer rats), rational actors optimize the signal and may actively work against the goal. The bounty measured dead-rat evidence, not live-rat reduction, and so funded the opposite of its intent.
The sewer system built to make Hanoi a sanitary, 'civilized' European enclave created a protected, interconnected habitat that let plague-carrying rats thrive and move beneath the whole city. The instrument of hygiene became the channel of contagion, so technological progress can manufacture the very hazard it was meant to prevent.
The plague and the failed rat campaign revealed that the racial and spatial segregation separating the French quarter from the Vietnamese population could not contain a disease carried by animals that ignored those boundaries. A regime premised on separating colonizer from colonized was undermined by a threat that treated both as equally permeable.