Goodhart's law
Often paraphrased as "when a measure becomes a target, it ceases to be a good measure." Named after British economist Charles Goodhart, who originally observed it in the context of monetary policy: any statistical regularity used as a control will tend to collapse once pressure is placed on it. The principle generalises far beyond economics — bounty schemes that pay per dead snake or per scalp incentivise farming snakes or scalping the wrong people; standardised test scores stop tracking learning once teachers teach to the test. The pattern recurs wherever a proxy stands in for what we actually care about.