NavList:

George H, you wrote:

"There may be an intended star, reflected as normal in the index
mirror, which may perhaps be confused with a brighter star, rather
overhead and somewhat behind the observer's head, seen through the
holes in the silvering. Can that happen, in practice?"

 

I have never seen a sextant that did not have an opaque barrier behind the mirrors, so, no, I don't think it can happen in practice. But on a related note, it can be very confusing when you're trying to see a star that is 100 degrees or more from the Moon. In that case, the image of the index mirror is highly foreshortened as seen from the horizon mirror so you can see a lot of sky behind it. It's not unusual to find a star in that direction, and it can be confusing. As you noted in a follow-up, there's never any 'real' doubt, but it's still a nuisance.