"Couldn't format create cell" warning can be triggered remotely for relayed cells
If relay process extent request then it can to fill logs by warnings about wrongly formatted create cells. It's easy to reproduce such cells and to trigger logs for relay.
if (r < 0) {
log_warn(LD_CIRC,"Couldn't format create cell");
return -1;
}
It need lower logs level or rate limiting or LOG_PROTOCOL_WARN (preferable) for relayed cells.