This makes phenomena such as a flow of energy from a colder to a warmer object so unlikely in the macroscopic world that their prohibition seems like a hard law, impossible to break. It is most likely that nobody will ever see something like this.

Quran, 25:53 "He is the one who has set free the two kinds of water, one sweet and palatable, and the other salty and bitter. And He has made between them a barrier and a forbidding partition"

Quran, 55:19-20 "He has set free the two seas meeting together. There is a barrier between them. They do not transgress"

There is no barrier at the boundary of different temperatures, the molecules can pass freely from one side to the other unless something else is holding them in place. What is important is how the movement of energy affects number of microstates on the either side as this tells how probable is such flow.

Tomasz Grysztar wrote:

There is no barrier at the boundary of different temperatures, the molecules can pass freely from one side to the other unless something else is holding them in place. What is important is how the movement of energy affects number of microstates on the either side as this tells how probable is such flow.

There is. The mixture of those joining molecules forms the barrier. Or else we gonna have salty river and blunt sea water. Neither transgress the other.