OCMock partial mocks crash when used with objc_direct methods that directly access instance variables. The code provided is a test case which shows the problem.
It's likely that nothing can be done about this, but it's worth adding a note to the documentation, in the section about Partial Mocks. In my opinion, this falls into the category of "There are some subtleties when using partial mocks."
MockingJayTests.m.zip