title: Xcode 5 says tests failed but shows green checkmarks
body: <p>I have written a few unit tests in the new XCTest framework of Xcode 5. </p>
  
  <p>The tests seem to pass successfully most of the time, but I've come across a strange behaviour where if I run the tests enough times consecutively, eventually I'll get a 'test failed' message and the console will show <code>Program ended with exit code 0</code>, but there are green checkmarks beside all the tests.</p>
  
  <p>What's going wrong here?</p>