This error causes a NodeMCU restart to occur. It occurs on expiration of the timeout value in the connection establishment, irregardless of the amount of traffic - in my case it's set to 120 and the clinet is publishing a topic every second, but after 120 publications it panics.
My assumption - the timer should be being reset upon a publication since that occurs 1 time per second, and should never exceed 120 seconds.
I can post my code if necessary, but is pretty straight forward classic - here's my substantiation of the client:
mqttBroker = mqtt.Client(clientID, mqttTimeOut, mqttUserID, mqttPass, 1)
where mqttTimeOut is previously set to 120
Versions: NodeMCU 0.9.5 build 20150318 powered by Lua 5.1.4
Any ideas or help greatly appreciated.