I used a /fill command to make a long box, at least 5000 blocks long, and it seems to have caused some weird lighting. Yes, the box is entirely enclosed, all sides but the bottom by stone bricks, and the bottom by grass/dirt.
Explanations of the screenshots:
In 2016-04-21_23.03.16: You can see some dark spots toward the back, whereas there's moonlight where the player is now. Everything is solid - there's no place for light to get in.
In 2016...23.10.01: Easier to see the different lightings.
In 2016...23.10.16: I thought that putting a light source might help things, but instead, I just ended up with a wall of light bordering on a wall of dark, instead of the usual fade one would expect.
In 2016...23.24.40: I put up a light source, like I did in the previous screenshot. Then I put one on the other side of the abrupt switch between light and dark. The light now fades normally. I broke the first light source, then the second, and the lighting still acts like the first light source - but not the second - is still up.