system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What exactly is Goodhart's law?

It states that 'when a measure becomes a target, it can no longer be used as a measure'. Any examples on that? How exactly is it used?
gpt
The head of an airport decides he wants to be able to brag about how quick he can get bags from the plane to the carousel so he can attract more airlines and offer a better customer experience . He says "The first bag is going to land in the carousel from the plane in under 5 minutes". 


That sounds great and a lot of people are interested so the baggage handlers are told that they have to make this happen otherwise they'll be fired the first crew to manage it 5 flights in a row get a bonus. For the first few days they struggle to meet this target. With the size of the airport, time to get the bags into the car to drive all the way across etc. 


So one crew decide to get clever about it. They get their youngest fastest team member. His job is now to jump on the landing plane, grab a small suitcase and sprint to the carousel loader with it. The rest of the team will debag the rest of the plane while he does that. Boom! This crew easily reaches the target. Other crews start doing this and suddenly travellers are waiting just as long as before but the airport can still claim to get the first bag off the plane and on the carousel in 10 minutes.

The kpi is now useless since it drives a change in behaviour as opposed to improvements.