I’ve reported a bug a while ago about this:
Bones that are facing roughly 160 degrees from their original orientation (in world space) suddenly flip the normals of normal maps in the faces they influence.
http://projects.blender.org/tracker/index.php?func=detail&aid=34061&group_id=9&atid=306
does anyone have a workaround in the meantime? or suggestions? .blend is attached to the bug report.