A few months ago I purchased a brand new D800 from a local store. I also purchased a Tokina 16-28 f/2.8 through B&H around the same time.
I am generally quite happy with the combination, but on about 3 different occasions I have been shooting in FX and all of a sudden I go to recompose a shot and I am in DX mode. For example, today I was taking a shot and upon the image review, I decided to change the ISO and aperture (shooting in A mode). I then went to re-focus the shot and I was in DX mode.
To resolve the issue, I turn off the camera, disconnect and reconnect the lens. The lens is then recognized as FX and everything is fine. I think it is also worth mentioning I'm mostly doing landscape photos, so I am not in settings such as continuous high (CH).
I am still getting familiar with the buttons, but I am not aware of any FX-->DX buttons that I would be bumping while changing settings. Has anyone else experienced this? Could it be the contacts of the Tokina? I have no other problems with any of my other lenses. At the moment is is a minor annoyance, but if it is an issue with the lens, I think I should probably send it in for repair.