Goodhart's Law
When a measure becomes a target, it ceases to be a good measure.Description
Goodhart's Law highlights the issue with focusing solely on the value of a measure as an indication of success or progress. Often, observers will artificially change their behavior to reach a target without caring about the reason for the measure. In other words, they'll try to game the system. The law was originally stated as: "Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes."
Examples
Google's PageRank algorithm would rank websites based on the number of incoming links from other websites. This led to webmasters gaming the system by artificially increasing the number of links to their website.
A company requiring writers to write a specific number of words a day will probably produce low quality, filler content as writers try to meet their quota.