Mobs (unsuccesfully) attempt to jump 1.5 blocks high.

Due to a bug in the AI, mobs treat slabs as a full block. When a "bottom half" slab is placed next to a wall 2 blocks high, it creates a 1.5 jump from the wall to the slab. But since the mob treats the slab as a full block, it thinks the jump is only 1 block high.