When a measure becomes a target, it ceases to be a good measure.
For example: my parents' health insurance company incentivized physical activity1 by giving rebates to people that got a certain number of steps daily, as measured by Fitbits. While my parents genuinely did make an effort to walk more, there were also days where they attached their Fitbits to the dog, gave them to someone else who was going for a walk, or even aggressively tapped their feet with the device on their knees while sitting in order to meet the measurement. Thus, their step counts ceased being an actual measure of physical activity.
How does this play out in your life, job, industry, field of study, etc.? What measures have been made targets? How has that changed the reliability or validity of the measures themselves?
Also, have you experienced any counterexamples? Are there measures in your domains that haven't succumbed to Goodhart's law? Why do you think that is?
1. This was the face value reason given. I'm more cynical and feel it probably wasn't about physical activity but instead about data gathering.