Computer killing powered USB hub. (Mac murderer)

This dubious hub has apparently killed three Mac motherboards so far, so let's investigate what could have caused that. Note that it's worth removing the hub and attempting to reboot the Mac, since the presence of the hub may be stopping it from waking.
First test was on the power supply with a PAT (Portable Appliance Test) doing an insulation test for a double insulated appliance between its input and output. It passed.

Next test was to see if it might be backfeeding the computer when it was turned off. The USB standard has a lot of protection, but I don't think they envisaged power actually being applied to the computer from a USB port. This one is backfeeding!

This is where an illuminated USB cable could actually show if a hub was backfeeding power out its input connection.

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