Sometimes when you modify a test in Visual Studio 2012, and then execute it via the VS Test Explorer, the test fails with the following error:

Result Message:    
 One of the known types provided to the serializer via 'knownTypes' argument was invalid because it was null. All known types specified must be non-null values.
    at ArtOfTest.WebAii.Design.ProjectModel.Project.LoadTest(String filePath)
    at Telerik.TestStudio.TestAdapter.TestExecutor.ExecuteTestCase(TestCase testCase)
 
Here's a video demonstrating the issue:
http://screencast.com/t/h2xgkYtOe0f
I'm able to reproduce this 50% of the cases.

The project seen in the video is attached to the internal bug description.