I think, this bug has nothing to do with the object parenting, it's somehow connected to the IK solver or bone -> follow object IK controller. I've experienced this also in Blender 2.49b. The bug appears when you are trying to change the bone transformation with IK follow object set in it. You can see in the attached file, that not only the object(rifle) is fluctuating but also all bones that are children to the IK bone, that is transformed (look at the left hand).
Tested on 2.76.