The only time I've had that happen is if I've created a shape using a plug-in and then open the file using a version of Carrara that doesn't have the plug installed. Were all objects created within Carrara or was one linked/imported? Hoof is right, it may be your lucky day and a one time corruption to the file happened. I'd create a fresh scene, drop in the arm and do a quick animation, close and reopen the file. If it happens again, I'd be in touch with Eovia PDQ.