kg4wsv:
A short will either shut the regulator down for the DC input or open the PTC (fuse) for the USB.
Yes, but that happens well after you hit the per-pin limit of 20mA.
It protects the regulator, or the device on the other end of the USB cable, but it won't do all that much to protect an overloaded output pin.
-j
That's why I specifically said "short" and not overload as you would have to overload all of the pins at the same time. Still a bad idea to do. BTW, don't the UNO specs say 40 mA?