This rig seemed to be going so well… then I opened it up and found it gone completely nuts. The culprit seems to be the left hand bone, called HandMain.L. This bone should be connected (and as far as I can tell it is) to the arm bone, but in pose mode, it’s not acting connected. On the contrary, not only is it moving completely independently of the arm bone, but it’s completely screwing up the whole rest of the armature. I’ve looked everywhere I can think of to see how it might be differently set up from the corresponding bone in the right hand, but I can’t see any difference.
Very grateful to anyone who can save me on this one!