towardsdatascience.com/unintended-consequences-and-goodharts-law-68d60a94705c
Goodhart’s Law is expressed simply as: “When a measure becomes a target, it ceases to be a good measure.” In other words, when we set one specific goal, people will tend to optimize for that objective regardless of the consequences.
A bit like if you are set a target as part of a goal, you do what you can to hit the target, and lose track of why it was set in the first place.
(Looking at you, Matt Hancock)