When you place 2 cobblestone slabs and make whole block, if you were to use pick block and place down the block you just got by picking the block, the texture would be double stone slab. I was on a superflat, singleplayer. World was created before 1.4.5.

How to recreate.
1. Place down 2 cobblestone slabs on creative.
2. Pick the block created by the double stone slab.
3. Place down picked block.