[bserge]

Most laptops and desktops will disable the USB port/controller or at least cut power to it.

It won't stop you from sending 200V into it and frying something, but it will stop short-circuits and faulty/non-spec USB devices from doing serious damage.

I guess it's a cheap safeguard that works pretty well for consumer products.

[avianlyric]

This won’t help you if the GNDs on both devices are different.

If the GNDs are different then neither device will be “providing power” in the traditional sense. Rather current will flow from the GND of one device to the GND of the other, and GNDs are normally not protected.

You will probably also see power flow on the V+ rails as well because they will be referenced against the GND in each device. In that case the devices can cut the power, but that only protects the V+ rail, not your GND rail that could still be transporting enough current to melt something.

[achn]

While modern PSUs are switch mode, surely there is still galvanic isolation from mains? Meaning both would have ‘zero equivalent’ ground, no?

[mnw21cam]

They can still be plugged into different ground supplies. If they have different potential, you could get a large amount of current flow. If they're plugged into sockets in the same house, this is unlikely to happen.

This is usually more of a problem with audio equipment that has analogue signals with very high sensitivity. A ground loop can convert any nearby magnetic fluctuations (like say from the electricity flowing through the mains cables in the walls) into a nasty bit of noise.