I had an issue where the foot rotated on my 600TC when I was carrying the camera and lens on a monopod. I was contacted by another member who mentioned that he had the same problem when he removed the hood and held on to the foot. I sent the lens in a few days ago. Nikon said it was a stripped socket and repaired it in a day. They're returning it to me today. I just wanted to let you all know of the potential issue and easy fix. No cost. They're also cleaning and inspecting the lens. A three-fer.
Marty
Marty