I purchased a factory refurbished 400mm f/2.8 AF-S (non-VR) lens a year ago from Nikon USA. Two major malfunctions happened. To me, both seem to be related to a little metal lever inside the lens's mounting mechanism that may have been a little bit off from its correct specifications, dating from the time I first received the lens from Nikon.
First of all, about two months after purchase, the lens stopped auto-focusing on my D7100 and other crop sensor bodies. This was the point at which I compared the 400mm lens to other lenses and noticed that there was a metal lever inside the lens's mounting mechanism that seemed a little bit out of kilter compared to other lenses. However, the lens focused perfectly fine on any full-frame body like a D800/E or D700.
About 91 days after I bought the lens (just after the 90 refurbished warranty period expired, of course), the 400mm lens became permanently affixed to my D800E camera body.
The second event happened by complete surprise, not from any noteworthy event such as a bumpy trip or excessive handling. All of a sudden when I tried to switch lenses, the 400mm lens would not come off. I have been very careful and not tried to force it, because this is about $10,000 worth of lens and camera that I am working with. So I have just had to use the D800E with the 400mm for nearly a year. So essentially I haven't been anywhere close to getting effective use out of my expensive investment in the 400mm, and have also avoided using it even when the D800E would be appropriate, because I don't want to make the stuck lens problem get any worse from further handling and using.
Has anyone had any experience with a 400mm lens become stuck to a camera body? If so, were you able to finally separate them without damaging the lens or the camera in any way?