CrossRoads:
If external power is supplied to the 5V header pin while the barrel jack is unpowered, the 5V regulator can be reverse driven & damaged. To avoid damage, connect a diode from 5V (anode ) to Vin (cathode).
That makes no sense.
For a start, the USB port does precisely that - supplies power to the 5V header pin and thereby the output of the regulator.