Minecraft 1.8.8, Minecraft 15w31c
When placing the slab next to the upside down purpur slab, I expected it
to stay visible.
Instead the side of the purpur slab became invisible and see through.
1. Place an upside-down Purpur slab.
2. Place any regular slab next to it on any of the four sides.
3. That side should become invisible.