Your unit test guarantees the intended interface of the unit, but it doesn't guarantee the usage of said interface. Typically other unit tests will stub out th

Your unit test guarantees the intended interface of the unit, but it doesn't guarantee the usage of said interface. Typically other unit tests will stub out this dependency. But what is your guarantee that the stub agrees with the spec'ed behavior. Now this may range from a non-issue if the interface is obvious and straightforward and leaves little room for error, to extremely error-prone in a duck-typed language. Of course there are ways to mitigate this with the test and stubbing infrastructure, but it can be brittle as well. This problem is why I think there is some sense of coarser-grained units with fewer stubs and some qualities of integration without going all the way up to the full-path acceptance level.