Hi, on 0.3.1 I'm seeing a placement bug.

Part one
Placing two slabs on top of a regular block is fine visually, but as per 'part two' of the report they will disappear.
Placing two slabs on top of a doubleslab or slab1/slab2 block causes it to either vanish and reappear in the inventory on the next place or causes the block below to be.
https://streamable.com/yjpvtd

Part two
Hitting any double slab block deletes the second slab, which reappears in your inventory the next time you place one down.
https://streamable.com/fpjrua

I snipped the start of my latest.log to where I started messing with the slabs:
http://files.theclassicpack.com/junk/latest.zip

Could you please send a mod list for the mods that you are using?

Sure, https://www.technicpack.net/modpack/the-classic-pack-beta/mods

This should hopefully be fixed in the latest version!