Correctness is not a binary thing though.

I doubt many people here are brave enough to claim their code does what is supposed to do in every conceivable case. Maybe you have high confidence in the correctness of parts of the code. Correctness of an application is murky though. Things we build are never fully correct, merely correct enough. Like maybe you're responsible for the UI in a web app and you're using your expertise to ensure it gracefully handles display across browsers and a gamut of screen sizes/form factors. But are you also verifying how it works when localized with an RtL script? Are you checking every change you make against CJK?