I wanted to reboot my pi today, so I pulled out the power Micro USB plug ... and nothing happened. The power LED stays on. Checking on the monitor confirmed that it was happily running along!
Apparently, the Pi can also be powered via the USB out, that's connected to a powered USB hub (although I think this is also a design fault in the USB hub). Or is this "intended behaviour"?
(and yes, I need to clean my desk)