Place a slab on the ground.
Place a diffrent slab above it.
Place the second slab but now ontop of the first slab.

The 3rd slab gets placed ontop of the other slab even though you don't placed it there. Before 1.3.1 you just couldn't place the slab. The bug excists since 1.3.1.

Link to the video: https://www.youtube.com/watch?v=PjobbDUnLSw