Isolated circuits: can their reference voltages (GNDs) be at the same potential?
There are few paradoxes in engineering because the real world doesn't have a lot of zeros or infinities, and the ones we create are handled by calculus.

It's not that current doesn't flow between isolated circuits.  If they were truly isolated, you'd be in Heisenberg territory, but in normal situations the resistance between them is very large.  Due to the high resistance, a voltage difference between them causes only a small, possibly immeasurable current to flow between them...but in order to measure this, you'd have to establish a connection between the two circuits anyway.

Whenever you find logical inconsistencies based on zeroes and infinities, they can usually be resolved by substituting "very small" and "very large."