No, it is that mocks can hide interface changes. So if you have a mock, then you need to test that the interface works without the mock. And if you are doing

No, it is that mocks can hide interface changes. So if you have a mock, then you need to test that the interface works without the mock. And if you are doing that, why not just skip the mock? foo calls x(user, date) foo mock # tests pass x changes to x(user, time) but the tests for foo do not change, tests still pass, runtime errors. If you have static/strong typing the compiler will pick this up – but for dynamic languages you have a problem.