Discovered when playing on my own survival server. What happens is you lose a full block of whatever you are converting.

Let me explain, you can store Iron, Diamonds, Gold, Lapis, Emeralds, Hay, and Coal into block forms. When this is done it requires 9 of whatever it is and it will give you 9 when you convert the block to the pure.

The bug comes into play when you take more than 7 (63 items) of these blocks and shift-click them into your inventory that has only 1 slot. It will give you a full stack of the resources with nothing more. Where you should receive a different amount.

For example: You take 10 Diamond blocks and you want to make a set of armor. You would have 90 if you do this normally. With a full inventory with a single slot open for the resources you would expect it to stop at 7 and give you 63. It does not, it converts 8 and only gives you 64 of the resource with a loss of 26.

I have tested in single, multiplayer. Creative and Survival. And with all of them (Iron, Diamond, Gold, Lapis, Wheat, Redstone, Coal).