At some point while playing, I noticed the night sky wasn't displayed correctly; it looked like the colors were inverted:

http://imgur.com/67wjuOg

When I exited the game and reloaded, the problem disappeared.

What's causing this? It looks like the game is reading the .ini settings wrong. It also happened to me once on another computer. And I have no mods installed, I just changed the Lighting values in the .ini file.