I believe the kind of scenario that bedobi is referring to is something like this (using your example): Unit test exists, ExampleFactory only calls ExampleServ

I believe the kind of scenario that bedobi is referring to is something like this (using your example): Unit test exists, ExampleFactory only calls ExampleService once. Hmm, it turns out that ExampleUIButton calls ExamplePoxyNavigator more than one time if the text in ExampleUIButton happens to be wider than the default width for the button. What does ExampleProxyNavigator do? Oh, it calls ExampleFactory which calls ExampleService. But only when the whole system is wired up for deployment. The unit tests indicate that the system should be functioning okay, but when you put everything together you find out that the system does not function okay.