Arduino Motor Shield - Voltage Supply

I have the following questions regarding the power of the Arduino Motor Shield:

Since the power of the Motor Shield is connected to the Arduino Vin (thereby fueling the board),
what happens if the Arduino is connected to the computer via the USB port with the shield connected to the Arduino?
It would be being powered by two voltage sources. It can damage the Arduino?

The mega has an automatic voltage selecting circuit that should only allow it to be powered from one source at a time . I would have to look at the schematic to be sure, but I think the barrel plug takes precedence over usb.