This lighting bug might be related to a couple of others I've seen that are open, but those talk about either skylight issues, or about lighting sources going "out" (like torches).  The cause of this bug could be related to the causes of the others, but I'll let a developer figure that out.

What: in a pitch black hole underground, put a single log into a furnace and smelted with a single plank to create charcoal.  Room lit up as furnace operated.  After furnace finished and went out, noticed that there was still light in the room.  Looked up F3 and server block light is 0, client light/block light is 9. 

Predictably, breaking the furnace updates the light.  Reproducable (see video).