You can find many simple but observed real world examples of unintended consequences from indexing on observation by searching for examples of Goodhart's Law (which, as commonly generalized, should actually be called Strathern's Law):

"When a measure becomes a target, it ceases to be a good measure."

See Wikipedia to get started, then google search for examples: https://en.m.wikipedia.org/wiki/Goodhart%27s_law