A ghast fireball that hits a roof made of (bottom) cobblestone slabs can do damage on the other side of the slab, even though it does no damage to the slabs themselves. That is, if a ghast fireball impacts such a roof, the contents of the room beneath it can sustain blast damage even if the roof is left intact.

This has been confirmed with cobblestone slabs (stone_slab), andesite slabs (stone_slab3) and stone slabs (stone_slab4), so presumably the bug affects every kind of stone bottom slab.

Top slabs (upper slabs) do not manifest this bug. Neither do full-size stone blocks.

Other sources of explosions also apparently do not manifest this bug (tested with TNT, beds in the Nether, and end crystals).

Steps to reproduce:

1. Import and open the updated attached world MCPE-106860.mcworld. You will find yourself in front of a button labeled Start with a stairway to its left and a room to the left of that with a ceiling consisting of cobblestone bottom slabs and a layer of orange stained glass below that.
2. Press the Start button. This repairs the ceiling (in case it is damaged from a previous test) and applies strong, long-lasting Regeneration and Fire Resistance effects to you.
3. Go up the stairs and out onto the roof. You will see two ghasts above you. Note: If either of the ghasts is missing, there is a button at the top of the stairs that will respawn it.
4. Move around the roof, especially into the corners, for 10–20 seconds while the ghasts shoot fireballs at you.
5. Go back inside, down the stairs and into the room. Examine the ceiling for missing glass blocks. If there are none the first time, repeat from step 2.

Expected result:
There are no glass blocks missing under the ceiling because cobblestone blocks are resistant to blast damage from ghast fireballs.

Actual result:
Some of the glass blocks were destroyed by fireballs even though the slabs are all intact.

Original Description:

I have a netherrack cooking facility and a collocated netherwort farm in the nether.  The building is made of netherbrick and is fully enclosed.  It is roofed with cobblestone slabs.  I often hunt ghasts from the roof while cooking netherrack and waiting for a new crop of netherwort to grow.  Ghasts are firing at me, hitting the roof and tearing up the netherwort that is 4 blocks below.