Nikon D3 viewfinder weirdness—followup

Following up on my Dec 22 entry regarding the “fireball” in the viewfinder, I’ve discovered what causes it: the Nikon anti-fog eyepiece! With the eyepiece in place the extremely bright spot is present; remove the anti-fog eyepiece and it disappears. I had used the as-shipped eyepiece until quite recently, which apparently does not have this issue.