Interesting. I looked at the datasheet for the regulator and it does indeed have an internal protection diode connecting Vout to Vin. It is capable of handling a 50A surge which is far in excess of what might be encountered in normal cycling of the device. In case the Vin is shorted to ground and Vout contains more than about 50uF of capacitance could this limit be exceeded requiring another external diode. I think reports of connecting a truly regulated 5V supply to the +5V pin causing damage are highly suspect.