Most USB hubs and active/amplified extension cables allow you to connect an external power supply to the downstream USB port so you can run devices that need more power than the upstream USB port can supply, and/or avoid any voltage drop from the lengthy cable.
Many of the cheaper devices however, simply connect the external power connector in parallel with the power lines in the USB cable. This means the external power supply can backfeed power up the USB cable and into the source device (such as a computer.)
Is this situation bad? I am guessing that if both the computer and external power supply are switching PSUs then they will simply adjust the amount of power each puts on the line and it won't cause any problems, but is this thinking correct? What if the external power supply is not of the switching variety?
Are there any situations where this could cause real problems?