initial situation:

    head -> 1 -> 2 -> 3 -> 4 ->null
                           ^
    tail ------------------|

        A executes, saves local copy of tail, context switch, A sleeps:

                           A: old_tail
                           |           5->null
                           v
    head -> 1 -> 2 -> 3 -> 4 ->null
                           ^
    tail ------------------|

        B executes, adds node 6 to queue, B done:

                           A: old_tail
                           |           5->null
                           v
    head -> 1 -> 2 -> 3 -> 4 -> 6 ->null
                                ^
    tail -----------------------|

        context switch back to A, A wakes up, A continues:

                              --------------
                              |            |
                              |            v
    head -> 1 -> 2 -> 3 -> 4 -|  6 ->null  5 ->null
                                           ^
    tail ----------------------------------|

  Result: node 6 gets "lost"!  Both processes think they have successfully
  added new nodes to the queue.