Goodhart's law, Campbell's law

When a measure becomes a target, it ceases to be a good measure.

Goodhart's law

Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes.

Paraphrased by Biagioli

When a feature of the economy is picked as an indicator of the economy, then it inexorably ceases to function as that indicator because people start to game it.

Campbell's law

The more any quantitative social indicator is used for social decision-making, the more subject it will be to corruption pressures, and the more apt it will be to distort and corrupt the social processes it is intended to monitor.