I found a MAJOR bug when using a Trowel with Quartz and using the numpad keybinds to change the type of block. It'll crash the client every time the world is loaded (or connected to a server). This basically keeps a player from playing that world/server until their inventory is fixed by editing files or an OP with FTBUtils can assist (that last one is interesting).

This was reproduced in a test pack with just BB and JEI ('cause everyone loves JEI); getting a trowel with quartz and pressing Numpad 3.

crash-2016-08-25_12.41.23-client.txt

Forgot to mention that I got the crash with Double Stone Slabs as well. Same steps, different material.