Cancelling recruits remaining from the previous turn resets resources, for example a resource 21 indie commander in a resource 12 province, if you bought him last turn and accidentally cancel him you'll have to restart or wait 2 turns again. I picked the indie commander example because that's the one I can think of having real significance.
In the same vein, from the moment I click (perhaps accidentally) on a mage/leader's "set battle orders" line, and for whatever reason his orders have not been copied, is there anything I can do to not cancel his orders?