You spent the week on a refactor because it was supposed to make the next change cheaper. The tests go green, the PR merges, and that's the last anyone thinks about it. Nobody comes back to check whether the next change actually got cheaper. The green bar said the code conforms to its spec, everyone read that as done, and the reason you did the work in the first place went unmeasured.
What green actually checks
Your CI is green. The test asserts assert response.status == 200, and it passes. It does not assert that the caching layer you just shipped cut p99 latency, or that the reworded onboarding step lifted activation, or that the prompt change made the agent follow instructions more often.
The green bar answered a question you never asked.
What did that green actually tell you? That the change conforms to the spec — the inputs map to the asserted outputs, the contract holds, nothing you wrote a test for regressed. That is correctness. It is real and it is necessary. It told you nothing about whether the system got better. You shipped because the bar was green, and the bar was never measuring the thing you shipped the change for.
