When I was crafting some smooth stone slabs, using 3 stacks of stone, I ended up with 5 stacks and 60 slabs. This was because I had i the middle of the process, not enough space to put it in and it had filled the last stack, but since slabs are created by the six, it had 4 left, and for some reason decided to delete them. I was able to repeoduse this by filling my inventory up so there was only room for 4 slabs, and when I created six slabs, two of them were lost. They were not to be found on the gound, nor in my inventory, not even when I left the world and re-entered it. I wasn't able to do this with furnaces.