The motor gets hot, particularly if you are stepping at a slow rate. I have accidently left 4-6x overvoltage on for awhile - the circuit breaker always blows in a few seconds on my power supply.
You can add current limiting transistor/resistor network to the driver circuit, but so far I have had no motor fatalities in a couple of years of using the simple overvoltage circuit I describe in altaz.zip.