[MC-127992] Light level 0 on slabs

Light levels were set to 0 on top of slabs, resulting in mobs spawning inside a well lit building.

Environment

Windows 10, Java version 8 update 161, approx. 12:30 a.m. in Norway April 5th 2018.

Duplicate of MC-46667.
And mobs cannot spawn on slabs. You probably have some dark space elsewhere in your house.

I checked after it happened, there were none. And none of the entrances were open.