What I expected to happen was...:
Mobs Like Chickens, Pigs, Cows, and Sheep don't escape in User-Created Pens as of 1.6.4.

What actually happened was...:
In snapshot 13w41b, I noticed that they escape even if I use Cobblestone Walls/Nether Brick Fences as substitutes.

Steps to Reproduce:
1. Build a pen at any size (as long as it is only 1 block high).
2. Spawn a Mob inside the pen.
3. Wait for a moment that mobs automatically escape their pens.