Persistent light (Not slab/stairs bug)

After turning off the redstone lamps in my 8-spawner mob farm, (2 spawners on each of the 4 sides), some areas remain lit. after locating the area of the fake light source using f3 I can place/remove blocks from the area and extinguish the light. Exiting completely and restarting also fixes the issue. Graphics settings such as smooth lighting have no effect on the bug. There are no other "real" light sources in the area. Changing gamemode and removing mods (T.M.I. and horsestats) has no effect.