What is the most expensive “small change” you made late in a project?

Late-stage requests often sound harmless: change the input flow, add controller support, adjust onboarding, rebalance one system, or support another platform.

But once a project has accumulated dependencies, a tiny change can trigger days of regressions and retesting.

What “small change” cost you far more than expected? What made it expensive: unclear requirements, hidden coupling, missing tests, or the number of environments you had to verify?