On Saturday, August 6th the CNC router malfunctioned, here are the details:
123 minutes into a job that I estimated would take 128 minutes, the machine moved to the corner closest to the computer and continued spinning. Mach stopped feeding code and the Stop button didn't work. The emergency stop button had no effect and turning off the inverter/opening the breaker was the only way to stop the spindle.
Mach was giving an error... something along the lines of "Z is beyond safe maximum Z"
Currently, turning on the inverter causes the router to spin up. The machine is unresponsive to any activity in Mach. Both the CNC router computer and the windows machine have been restarted. The bit has been removed from the router.
My amateur guess at the cause is dust in the computer. I imagine the router computer will need to be reset and it might not hurt to clean out the cases.