Summary: Ever since 15w49a when mob pathfinding was improved, mobs ignore the void, thinking it is NOT an obstacle, causing the mob the fall into the void. Took me a long time to confirm this.

To reproduce:
1. Make a world with the superflat preset: The Void.
2. Make an area where a mob will try to kill another mob (e.g. Zombie killing Villager) or where a mob is trying to attack the player, can't be a ranged mob.
3. Make a gap between the mob's location, and the villager/player's location, this gap should lead to the void.
4. Spawn the mob / villager in the area, or get into survival if the mob is supposed to be attacking you.
5. Normally, mobs will stop at the gap if there is blocks, but since it is the void, the mobs will simply go through, or if the mob has to walk on a specific path, to get to you, the mob may take a shortcut, by attempting to walk on air, but can't because the mob simply falls into the void.

NOTES: If the reproduction steps are incoherent, see the screenshots to get a better understanding.
If this is a duplicate, I have already tried searching, but didn't see my issue.