On Thursday, 19 May 2016 11:37:06 UTC+1, News wrote:
In message , Dave W
writes

Another thing I was careful to avoid is non-flat glass, which I often see.
It doesn't affect the view through it, but the reflections off it make it
look like plastic windows on a dolls house!

Tangent alert! Our windows are all still original Victorian sash, and
many panes (with flaws) are probably original, too. One bedroom window,
at the front, is odd. Traditional sash, with four panes of glass, yet
one of the four always looks as though it is not there, from outside. It
just looks dark, and you would think the whole pane was missing. Looking
out, from the bedroom, it looks exactly like the other three. No idea
why.

If it is a different/float glass it could look like that especially as the optical qualities or older glass wasn't as good as float. The only other reason I can imagine is the plane viewed from outside which would obviously change as the viewer moves.