A "strange phenomena" occurred when I was making an illustration for this discussion.
I must have accidentally changed the time from day to night (which I would presume should not have any effect when rendering an image in a room without windows), and all of a sudden my ceiling had gone black.
And not only that, as you can see:

Daytime:


Night:


I tried changing the settings, but it had no effect.
The ceiling goes black when the sun goes down:



Is there any rational explanation for this peculiar abnormality?

Cec

edit:
I have a link to the sh3d-file here .