Goodhart's Law

The Goodhart's Law on Wikipedia

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

Charles Goodhart

Also commonly referenced as:

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

Marilyn Strathern

The law states that the measure-driven optimizations could lead to devaluation of the measurement outcome itself. Overly selective set of measures (KPIs) blindly applied to a process results in distorted effect. People tend to optimize locally by "gaming" the system in order to satisfy particular metrics instead of paying attention to holistic outcome of their actions.

Real-world examples:

Assert-free tests satisfy the code coverage expectation, despite the fact that the metric intent was to create well-tested software.
Developer performance score indicated by the number of lines committed leads to unjustifiably bloated codebase.