Calling teardown on the UnityContainer doesn’t remove a registration from Unity.  Here’s how to unit test code that resolves a type configured by RegisterInstance rather than configuration.  IoC is a thin wrapper around the UnityContainer.

        private ControllerContext _controllerContext;
        private ModelMetadata _modelMetadata;
        private Proposer _proposer;
        private ILookup _lookup;
        private LifetimeManager _lifetimeManager;

        [TestInitialize]
        public void SetupTest()
        {
            _controllerContext = new ControllerContext();
            _modelMetadata =
                ModelMetadataProviders.Current.GetMetadataForType(() => _proposer.AcceptedTerms, typeof (bool));
            _proposer = new Proposer();
            _lifetimeManager = new ContainerControlledLifetimeManager();
            _lookup = MockRepository.GenerateStrictMock<ILookup>();
            IoC.UnityContainer.RegisterInstance(typeof(ILookup), _lookup, _lifetimeManager);
        }

        [TestCleanup]
        public void VerifyExpectations()
        {
            _lifetimeManager.RemoveValue();
            _lookup.VerifyAllExpectations();
         }       

        [TestMethod]

        public void ErrorMessageIsCorrectlyGenerated()

        {

            var target = new NotDefaultIfAttribute("test/notDefaultIf")

            {

                OtherProperty = "Surname"

            };

            _lookup.Expect(l => l.GetLabel("test/notDefaultIf")).Return("Blah blah {0} blah");

            Assert.AreEqual("Blah blah AcceptedTerms blah", target.FormatErrorMessage("AcceptedTerms"));

        }