Well, yeah, now I see what you mean, but only when I enlarge the pic to something like 160%. OTOH, that's not what I (and I think AS) meant. I am talking about HDRIs representing an enclosed space, not about HDRI being visible in an enclosed space. In fact, I think what you discovered is a bug. If the HDR is completely outside the box - while your sphere and the camera are inside - there shouldn't be anything visible in the box. For my example, see St. Peter's probe and Grace Cathedral probe from debevec.org or check http://www.worldof3d.com/tutorials/tutorial8-01.html for the panorama.

-- erlik