Incorporating a locked unit into a division (using Ctrl+c, you can't do it with the button) and then removing it again unlocks the unit. This is especially dangerous when combined with the redploy/undeploy exploit. One commander can form a division on turn one or two, and then bounce around the map freeing all the garrisons. And he doesn't even need to stay a division commander at the end of the turn if he doesn't want to.
"Poof! Hi, I'm Steve Hurlburt, your new commander. Be Free! I now resign as your commander. Got other things to do. See ya! Poof!"
I'm calling this one the fairy-godmother exploit.