Should Mock Objects be considered harmful?

Robert Collins asks the question:

Should Mock Objects be considered harmful? As an optimisation for test suites they are convenient, but they mean you are not testing against the something which can be verified to behave as the concrete interface is meant to, which can lead to Interface Skew.

Let's says we have an object A which uses an object B. In OO A and B represent roles: object A does something in collaboration with B. A behaves "right" only if B behaves "right". So we have a behaviour contract between A and B. This normally represented by some unit-tests for the B role which specify its behaviour. Based on that, we can test A: we "mock" the B role, and see how A reacts. If we want to keep our implementation clean, A knows nothing about B implmentation, it knows only about its behaviour. (B is an interface, or A and B are implemented in a dynamic language).

Once we have tested B's behaviour and A's behaviour they "should" work together without errors. This doesn't happen: usually B is not throughly tested, the behviour contract is broken. for this case we should have integration-tests: A and B play nicely together.

Testing.Kata: unit-test role B, unit-test role A, integration test A and B.