You should see what happens when people jam their ac-adapter plug into the USB port.
Lets just say certain chips and circuit paths on the motherboard don't take too kindly to that.
As far as these devices go, all you really need to do is cross some contacts in the USB port to fry the motherboard.
For a while at work I had a USB to hard drive adapter, that had a power supply for the hard drive that you would plug into the adapter to power the hard drive. If you did things in the wrong order, you'd kill the motherboard.
Plug in power, make sure drive is powered on,
then and only then plug the usb end into the shop computer.
Killed a couple shop computers before I figured that out.