Mining Hardened Clay with Pickaxes doesn't drop the item.
The same goes for Slate from earthworks.

Edit: Redstone Blocks, too.

And tinkers' smeltery blocks

That's quite odd. I'll look into this for the next version.

@the-realest-stu I experience the same problem with the stones added by Quark (Limestone/Marble), blocks chiseled by Chisel. The WAILA Harvestability add-on says instead of a red X or green Check, too.

Bone blocks also get destroyed. Normal Minecraft pickaxe can collect them.