What I expected to happen was...:
The Mobs would not allow themselves to fall into the void.

What Actually Happened was:
The mobs Fell into the void. (watch the whole video)
http://www.youtube.com/watch?v=77U7k1tRjkE&feature=youtu.be

Steps to Reproduce:
1. Dig down near the void, make sure the void is accessable.
2. Spawn a mob on a block
3. Wait for it to move, or make it move (without pushing) into the void.

It happens because the Mob AI looks for air blocks, But there are no air blocks in the void.

DUPLICATE OF CLOSED ISSUE MC-3151, The issue was labeled as "invalid" I now have proof that it is not invalid.