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.
MC-66114
Mobs (unsuccesfully) attempt to jump 1.5 blocks high.
Resolved
Fixed
3
(Unassigned)
Community Consensus
Minecraft 14w32b, Minecraft 1.8.3, Minecraft 1.8.7
Minecraft 16w05b
Retrieved
Is this still a concern in the current Minecraft version? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases. If this has been done, we can reopen the issue.
Keep in mind that the "Resolved"-Status on this ticket just means "Answered", and that we are waiting for further information on whether this issue still exists or not. We will reopen it as soon as the requested information has been delivered.