The question has come up before. The answer is that if you push the power button once nothing should happen but if you push 2 times fast it signals an emergency shut down. The car will need to be pulled off to the side of the road to a stop and restarted.
I haven't done this by accident (the feel of the power key is pretty different than the drive mode key), but I did this on purpose to see how the recovery was. No big deal. Shift to N and hit the power button to restart. It is not necessary to stop the vehicle.