i found a bug: make cobblestone slabs, in a crafting table put 2 slabs one on top of another. and you get the 2 smooth stone slab block, see pictures in linked zip file.
https://www.dropbox.com/s/ttp5rvdp2a30zyq/FTB slab bug.zip
edit: i have no idea where to post this i'm new here.