This code here will hide the true error of certain failed async tests:

https://github.com/mochajs/mocha/blob/master/lib/runnable.js#L216-L230

To reproduce, try this:

it('should work', function (done) {
  setImmediate(done)
  throw new Error('hidden')
})

You will never see the error with the "hidden" message. It will just tell you done was called twice, which is terribly misleading, as the user code only called it once.