minecraft-java-edition
I built an underground mob spawner in 1.8 SMP that I can turn the lights on and off in. I used pistons to move glowstone over glass holes in the ceiling – whenever the lights are off the holes are covered.
But…there's light coming in from somewhere! The rest of the room is solid cobble or smooth stone, so I can't figure out why there would be so much light! How can I seal the room in darkness?
Different angle:
A fully modular system inside a 3x3 or 4x4 (with 1 block/1 lamp per level) is impossible because of the lack of space to fit repeaters or redstone torches in a configuration that will still respond correctly to an on/off switch.
The tallest tower that I could make respond correctly to an on/off switch was this 3x3
Which uses an upside-down half slab in the middle and next to the unlit lamps and is powered by a redstone torch underneath a solid block (instead of a half-slab) in the middle of the bottom level
The smallest size of free standing tower that I could fit a fully modular system in was a 3x3 with 2 blocks and 1 lamp per level, controlled from the bottom
This is the first part of the control system (there's a redstone torch on the inside side of the block with the torch under it)
This is the second part of the control system (there's a solid block in the middle of the cross)
If you built the tower against a wall however, it is possible to fit a fully modular system inside an apparent 3x3 like this
Sounds to me like a shading issue where Minecraft is just trying to make stuff visible, but the lighting is probably still nil.
If you press F3, you should get some text in the corner of your screen. Find which one is for the light and see if it is the same regardless of your visibility. If it is, then this is simply Minecraft glitching on the shaders and you should be fine.
Best Answer
I saw this bug in some version of 1.8, I think (but in the form of torches placed underwater leaving light behind). I didn't find any way to fix it.
It's fixed in a 1.9 prerelease.