A ghast fireball that hits a roof made of (bottom) stone slabs can do damage on the other side of the slab, even if 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:
(A demo world is attached.)

1. In the Nether, build a structure with blast resistant walls and give it a roof of cobblestone bottom slabs.
2. Fill the layer below the roof with blocks that have low to moderate blast resistance.
3. Go onto the roof in survival mode and lure a ghast to shoot a fireball at you. Step aside to allow it to strike the roof instead.
4. Examine the low-resistance layer in the room.

Expected result:
Wherever the roof was left intact, the fragile layer below it is also intact.

Actual result:
Sometimes the fragile layer is damaged even though the roof above it isn't.

Note: MCPE-29200 fixed a bug whereby ghast fireballs could destroy cobblestone slabs and walls. It's possible that the fix was incomplete in that it prevented the damage to the cobblestone but didn't stop the logic that follows the propagation of blast damage through it.

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.