The extreme isolation of the tests, combined with the fact that they are not usually deployed, often causes developers to think that tests fall outside of the design of the system. This is a catastrophic point of view. Tests that are not well integrated into the design of the system tend to be fragile, and they make the system rigid and difficult to change. The issue, of course, is coupling. Tests that are strongly coupled to the system must change along with the system. Even the most trivial change to a system component can cause many coupled tests to break or require changes.
