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.
Duplicate of MC-1959 , please use the search function to see if your bug has already been submitted. Currently over 30% of tickets are being closed as duplicate.