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.
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I have duplicated this in 1.7.2 and 1.6.4 superflat worlds by creating large stone structures with roofs made of redstone lamps.
Note: All the fake light sources exist along the chunk boundaries. {X and Z coordinates c: (0, 15)} This light source is equivalent to the light level at that location when the lamps were on. many of these fake light sources were several blocks away from the structures.
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Ok, despite that fact that these fake light sources can fool me visually and even show up on the debug screen, they don't fool the zombies. I enclosed one of these apparently well lit areas with stone and dropped a zombie spawner on the ground. With the debug screen showing the lowest light level in the room as bl:9, zombies spawned as if the room were dark.... The zombies seem to be getting smarter. 🙂
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Can we mark MC13003 as a dupe of this one? I can't adjust the affected versions there and it clearly affects 1.7.2 as well.
@Bob, your zombie spawner observation confirms also that this is a client-only issue, the server actually has the correct light levels. I have uploaded my witchfarm as a world download to the other ticket, but a more isolated case might ease finding the cause of the bug for the developers. I haven't succeeded in creating a simpler case, but it sounds like you might have. Could you by change attach a world download to this ticket?
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Uploaded my test world for the light bug. zipped using 7zip.