Nether brick
I attached a picture to show what I mean.
My apologies, there is also a half slab under where the pig's stand. So there is a half block space above their head. This used to suffocate them in 1.7.9 though and does not anymore.
So this is intentional and is staying? I've spent the last few months working on slabbing the nether for a wither skeleton farm that will now be useless?
My server is also randomly closing itself in 14w32b