Jumping bones

Jumping bones is the (presumably colloquial) name of a little-understood medical condition which causes uncontrollable jerking movements in the sufferer. It seems to be exacerbated by the nervous stress of having to carry or handle fragile objects. Coachman William Carpetlayer was afflicted with the problem until cured, or at least temporarily relieved, by Tiffany Aching (unfortunately too late to save $1000 worth of delicate cargo).