This is a bug involving stairs, slabs and smooth lighting.

So basically, when a stair block or slab is placed in a certain way, instead of smooth lighting working normally, the stair/slab makes the rest of the block pure black or really dark.

This has been an issue for quite a long time now, and Mojang attempted to fix it ONCE but then took away the patch that fixed it for some unknown reason. But I'm thinking that instead of releasing something new in future snapshots, how about we take care of one of the longest running bugs in Minecraft; the lighting bug/glitch!