Snapshot 13w06a, Minecraft 14w04a
Tested on two computers when reported:
1. Windows XP
2. Windows 7
Both had most recent Java, LWJGL.
As of update for 14w04a, has been tested on a computer with the following stats:
Windows 8.1 64-bit
I noticed this many versions back when I was trying to figure out lighting for a large room. Even if it is night time, if you open the debug screen with F3 and look at the sl and rl values, they always say "15".
Edit: Although this has not been updated since 13w06a, it has existed in all intermediate versions through this update (14w04a). Another screenshot is also available in
MC-45473, although the bug there is different.