The attached error occurs when I select the figure's root bone and then hit copy followed by paste.
The obvious use of Mirror Copy is to "flip" a pose (e.g. if you're creating a walk cycle, you start by creating one pose as a keyframe, making two evenly-spaced copies, then flipping the middle pose. But when I try to do this I get an error.
I can flip parts of the pose (e.g. copy the left arm's pose, and paste a flipped version on to the right) but this doesn't help much -- to get the desired result I need to flip each limb individually, and use a stored pose to get the unflipped data back (hard to explain... but it's not pretty).