Sense resistance based current sensing
It is not uncommon for equipment to share a ground connection. Communication protocols, analog I/O, and power adapters generally do. Medical applications are one of the few exceptions. However it is rare for equipment to share a power connection.

If, due to a fault or design oversight, power finds a path of conduction through one of those external connections then current will flow through a ground that is not part of the low-side sensing circuitry. 

I’ve seen this happen with USB ports in computers, when the equipment that they are connected to used the wrong kind of power adapters (e.g., a non-isolated one, when the equipment specified an isolated adapter).