There is an adiabatic box, with a board in the middle, which is adiabatic too.The board separates the box into two parts and there is a small hole in it, which is close to a coil and the hole has a little door, designed to be opened when the current in the coil reaches a certain value.
Now, if I put some magnetized gas in the box, through the right side, the gaseous molecule, which has the larger velocity, when it passes through the coil there will be current produced in the coil, so the door will open, so then the molecule can pass through the hole to the other side.
After some time, the faster molecules will come to the left side and the slower molecules will be left on the right side so the entropy in this isolated system increases spontaneously. Does this violate the second law of thermodynamics? Of course not, but what was the problem?