The bug

Certain blocks "save" block light, emiting it even after the original source has been removed.

Affected blocks

• Lit furnace
• Lit redstone ore
• Magma

Reproduction

1. Build a sealed room without any lights in it with a single layer between it and the sky.
2. Use /setblock to create one of the effected blocks.
3. Measure the value of block light using.
4. Put a glowstone block right next to the effected block.
5. Break the glowstone.
6. Observe that the lighting value (as seen in F3) has changed.

Notes

• All of the affected blocks are full cubes that emit less than 14 block light. (Non-full-cube blocks such as brown mushrooms (1 light) are not affected, and the effect isn't observable for bright blocks)
• The resultant light is 2 less than the light value of the block you placed next to it, which is 13 if glowstone is used. This is because the block receives (and thus emits) the light level - 1, and then another - 1 occurs due to going back to you.
• This behavior randomly decays
• This behavior when using magma blocks is fairly popular for builds (as it is nice for decorations). If removed, it may be beneficial to introduce a similar behavior within the block's state and use an upgrade path to store the current light.