Control Module was Removed from Array Error

We have an EqualLogic PS6000 running firmware v8.1.11. It is part of a group of 7 arrays, all running firmware v8.1.11.

The member referenced above sent a series of Error and Warning messages.

The first warning was "Control module was removed from array". This was followed by warnings the controllers cannot communicate and control module failover cannot occur; etc.

In a couple of minutes there were messages that a controller module has been installed in slot 0, the controllers were communicating properly, the control modules were initializing, and failover was operational.

This occurred a few days ago and the drive array has not experienced any additional errors. Does anyone know of a firmware bug or failure mode that could be the cause of this behavior?