What is?

Where in my comment did I say anything about manual process? Please re-read what I wrote.

> What do you do when that happens with "regular" unit tests that test every single piece of code in isolation? > You fix it.

If you don't find it via an automated means, it's via a manual means. The implicit assumption is that the error is found, regardless of where that happens, in dev, test, prod, or your env de-jour. Good luck with whatever.