gistfile1.txt

class Voyage
  def prepare_voyage(vessel)
    vessel.prepare
  end
end

class Vessel
  def prepare
    .. do stuff ..
  end
end

.. test ..

class VoyageTest
  test "should prepare a vessel" do
    os = OpenStruct.new(:prepare)
    assert Voyage.new.prepare_voyage(os)
  end
end

## If we update vessel to not have a prepare method ##

class Vessel
  def not_prepare
    .. do stuff ..
  end
end

Our unit test for vessel will fail because prepare will no longer exists but we caught it and we can fix it.
Our unit test for voyage will pass because we are passing in a os that responds to prepare.
Our production work will fail because vessel no longer responds to prepare