def cmd_ERROR(self):
        """
        A non-existent command has been sent.
        """
        log.err("Non-existent command sent.")
        cmd = self._current.popleft()
        cmd.fail(NoSuchCommand())

        A non-existent command has been sent.
        
stackoverflow/venv/lib/python3.6/site-packages/twisted/protocols/memcache.py