The same has happened to me once or twice. I selected Shut Down through the Charm menu, and it appeared to shut down. Then a bit later, my kid came along and plugged in a USB fan to the "shut down" SP3, only to have the fan actually power up. I have verified since, that the USB port does stay powered when in sleep/connected standby (yay!), but does not when shut down.
I paid attention a few times after and noticed it happen at least one other time. But haven't been focusing on it too much since. Also, I can't remember if it's happened since I did a full restore.