I was crafting some stone slabs and noticed I only had one slot left in the stack. The rest of my inventory was filled. When I shift clicked on the slabs it used up the items. It then added 1 slab to the stack of 63 in my inventory. It couldn't have made a new stack because the rest of my inventory was full.

This works with any multi-crafted item and works on servers or single player worlds.

I have tested this glitch on the following items:
Slabs
Sticks
Iron (from iron blocks)

To recreate this simply:(using slabs)
Fill your inventory up with an item other then slabs
Put a stack of 63 slabs in your inventory
Attempt to craft 6 more slabs