If you have a stack of 64 blocks of __ (for this example I used Blocks of Redstone) and not enough open spaces in your inventory to fully break the blocks into dust it will fill the open spots with the dust and cause some dust to vanish. In screen shot 1.png I have the full stack of blocks in the table and only 7 open slots in my inventory. Screen shot 2.png is my inventory after shift + click the dust in the crafting table's output slot. Screen shot 3.png is a photo after the consolidation of the sticks in my inventory. Screen shot 4.png is the result of shift + click the dust in the crafting table's output box. Overall the result is a loss of two dust.