When two cobblestone slabs are placed one on top of the other in the same block space, they form a block that has the same texture as the Stone Slab. When broken, the block drops two cobblestone slabs again. (This does not happen when two cobblestone slabs are on top of each other but in different blocks, however.)

What I expected to happen was...:
Two cobblestone slabs form a cobblestone block when placed.

What actually happened was...:
Two cobblestone slabs make a new block that is full-sized but with the stone slab (from smooth stone) texture. All cobblestone slabs that had been placed on top of each other before updating to 14w17a converted to this new block type. (I haven't yet made a pickaxe with Silk Touch to test whether this new block is harvestable; breaking it otherwise drops two cobblestone slabs.)

Steps to Reproduce:
1. Put a step by step guide on how to trigger the bug here
2. ...
3. ...