My point was, there is no way to guarantee bugfreeness in any way. You can reduce bugs by automating and another test layer, but each new layer can have bugs to

My point was, there is no way to guarantee bugfreeness in any way. You can reduce bugs by automating and another test layer, but each new layer can have bugs too. Even if you prove the code to be correct, you only proved your assumptions of the requirements which might be wrong (both your assumptions and the requirements). Because the comment said something like: There was a bug, so they didn't have a test, why don't they? And I replied: Tests can have bugs too.