Every chunk is made up of several sections, or so I have heard. When all of a section of a chunk is air blocks, the game seems to forget about that section and doesn't update its lighting. So, every air block in that section gets sunlight.

This can be a problem in large caves in normal Minecraft worlds but can especially be a problem in custom maps where large areas are common.

Tested with and without the OptiFine mod installed. Has been happening since the introduction of the Anvil world format in 1.2. This is not caused because there is a hole to the void.

To recreate, simply generate a superflat world and create a box around the player that is at least 30 or so blocks tall and a chunk large. Sunlight will most likely appear in the box all the way down to the floor.