Goodhart's original formulat

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

Mario Biagioli has related the concept to consequences of using citation impact measures to estimate the importance of scientific publications:

All metrics of scientific evaluation are bound to be abused. Goodhart's law (named after the British economist who may have been the first to announce it) states that 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.