Any observed statistical regularity will tend to collapse once pressure is placed upon it for control purposes.
Named after economist Charles Goodhart.
Rephrased and generalized by anthropologist Marilyn Strathern as
When a measure becomes a target, it ceases to be a good measure.
Essentially: if you use something for reporting, people will modify their behavior to make it useless. An indictment of KPIs, etc: "we need to reduce the number of tickets not done in a sprint" becomes "only pull a ticket into a sprint if it's done".