code: public synchronized void mapOutputLost(TaskAttemptID taskid,
                                           String errorMsg) throws IOException {
      TaskInProgress tip = tasks.get(taskid);
      if (tip != null) {
        tip.mapOutputLost(errorMsg);
      } else {
        LOG.warn("Unknown child with bad map output: "+taskid+". Ignored.");
      }
    }
docs: A completed map task's output has been lost.