Goodhart's Law

Goodhart’s law: “Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes.”

And Marilyn Strathern’s generalization of it: “When a measure becomes a target, it ceases to be a good measure.”

What are some examples of Goodhart’s Law that you see in the world?

Definitely interest rates vis-à-vis the federal reserve. Let me mull over some others