If I go and light up a cave whilst not in the spawn chunks and leave the area the game will require me to break and replace the torches to register artificial light, this is the same for the sky light. If I leave and return to the chunks the lighting engine will not register any blocks that I have placed as valid sources of protection and will even disregard any naturally generated blocks as valid sources of light blocking, this has broken a skeleton farm I have created and slime farm near to it as the light level drops to 0. and other mobs spawn, or the other way around the sky level rises to 15 and no mobs will spawn, I have combated this in game by breaking and replacing the blocks when I need to use either of these farms but is a major issue that is affecting the whole server; however, I have not run into the bug on my single player worlds. reguards GobbleTheEski