When /hat is used, it will replace any item in the player's head armor slot with the one currently in the main hand. This can be somewhat exploited to unequip helmets with the Curse of Binding. It would be nice to have an option to prevent this, but of course there's the argument that players do not necessarily need access to /hat in the first place.

I imagine this could be easily taken care of with a sub-permission (given by default).