If you open a test runner and select only KernelTests-ClassTest and TraitsTests-TraitsTestCase you may find a failure ClassTest>>testRootsOfTheWorld But I don't understand fully why it appears. I had once the chance to get a debugger on the failure (which does usually not work on this failure) and so I know that the rootsOfTheWorld (which is usually 3) is 6 in size containing additionally classes C3 C4 and C6. The ones which are created by TraitsTestCase (see my last mail "Bug for harvest"). They all have a superclass of nil but they aren't included in the SystemNavigation. (conditions for being included in the rootsOfTheWorld) If you press "Run selected" again the same error occurrs while pressing "Run failures" gives you all green (ok, it is this way in my case) And I can't reproduce the same in a 3.10 image. It seems that there is a timing problem involved. Selecting only the two test cases I get the failure every time (or nearly). Selecting all tests it appears only at random. Any hints? Norbert