Maybe I can clarify a bit further. If you ever work with sound systems, you will quickly learn that the hum from the sound equipment is due to mismatched ground potential between sound components. In this situation, you make sure the system grounds have nearly zero resistance to prevent this audio ground loop current that results in speaker hum. Ground loop currents exist in our physical electrical world and sound systems are a classic example. These small audio ground loop currents do no damage because of their small milli-volt differences involved. They just annoy. AC mains ground differences on the other hand are not protected by circuit breakers and can provide significant amperage when they exist. The further you get from the building power input panel, the greater these ground voltage differences can become.
When you pass several amps of AC current through the Nano ground path as in the above post example, this current can end up passing through the Nano circuit board via the ground foil (trace). In this example the ground current enters on the Nano test lead ground, and exits on the Nano USB jack ground. These two points in the Nano are connected via the Nano circuit board. The traces on this board were never designed to pass this volume of current so they can heat up enough to lift from the circuit board and move to who knows where before they burn open.
If you are lucky, the hot lifted circuit foil just burns open and the ground current is stopped with just a burned open ground path on the circuit board. In this case just repair the opened ground foil and you are back in business.
If you are less fortunate, this hot lifted circuit board trace moves enough to touch other circuit board components and shorts them to ground while the Nano is still providing circuit power and before this trace burns open. This is usually catastrophic with respect to circuit board damages.
This same business takes place inside the laptop USB port. Those traces are also subject to the same heating effects from this ground loop current. In the laptop the ground loop current flows from the USB jack ground pin via the circuit board to the power adapter jack ground pin, via the power adapter to the wall outlet ground.
I refer to all of this as ground loop current. So the Nano sees this ground loop current and so does the laptop USB port. Any potential difference between two points will provide current flow when a path is connected. AC mains ground voltage differences are no exception. This AC ground potential difference will in fact pass through any ground path connected between this potential difference.
Hope this clears things up. You got a very high return for your three words. 