Certain chunks in a world will became dark when it should be lit after you reload a world.
Steps to create this bug:
1. Create a big room, make sure it blocks sunlight
2. Place torches, preferably more than 10, spaced at least 10 blocks around the room
3. Exit the world, and join in again.
4. After trying a few times, some of the light will not be rendered correctly, and areas just beside a torch will be dark.