* the putIfAbsent returns a non-null value, and the case where the replace() of an observed zero
   * fails.
   */
  public void testAdd_withFailures() {
    AtomicInteger existing = new AtomicInteger(12);
    AtomicInteger existingZero = new AtomicInteger(0);

    // initial map.get()
    when(backingMap.get(KEY)).thenReturn(null);