OK, this is a weird one. I've heard that the two USB ports on a MacBook Pro can be different with regard to data transfer rates, but this is another oddity
.When I plug my USB cable (with an adapter on the end) into the rear USB port to charge my phone, it works fine. When I plug the same cable into the front USB port, the machine turns off. Every time. My understanding is that this power-down is to protect the machine in the event of a detected short. OK, so I must have a short in that cable, sort of, maybe, and the USB ports on that machine somehow have different sensitivity to such shorts.
But then I try the same thing in another Mac Book Pro, and it does EXACTLY THE SAME THING.
This is, by the way, without the phone attached. Just the cable.
So there is evidence that this asymmetry is not just one machine, but a property of the machine architecture.