Steve,
You are right there can be numerous of sources of glare, but the most common I've seen are either shiny internal surfaces, which can form a partial or total halo around the exit pupil (a production problem). Or alternatively the spurious reflections we usually call false pupils if they encroach on the exit pupil (a design problem). As our pupils dilate in low light both are increasingly likely to interfere with the view and centering the view becomes increasingly more critical if problems are to be avoided. It's likely to be more problematic with the 4.2mm EP of 10x42 than the 5.25mm of an 8x42.
I believe the ELSV 8x32, for instance, is a false pupil problem, but Franks original complaint about the Nikon M7 8x30 was shiney surfaces (which I've not encountered). However, the M7, Kite Lynx, and I've read the Maven B3, also have false pupils just outside the EP, which I've found can often be avoided with care.
I'm just asking if Peter, and others if can do a bit of diagnosis to understand if it's a manufacturing glitch or a feature the design, and to what degree it can be avoided.
David