I would say that the best economic value is being delivered if we consider some code fragments that are almost never covered by tests (any of them) - like error

I would say that the best economic value is being delivered if we consider some code fragments that are almost never covered by tests (any of them) - like error detector. Guys at pvs-studio brought in some pretty funny example of such errors: http://www.viva64.com/en/a/0078/ http://www.viva64.com/en/a/0078/