I made a pen to keep my animals in (a square area of fences) and they seemed to all huddle into the corner of the pen. I could not use my player to push them out because they were in the very edge. The animals either couldn't or wouldn't move out of the corner (even when i tried to bait them out with their corresponding breeding material) When i bred some animals, the baby's would grow up and escape the pen.
Mobs effected: Pig, Sheep, Cow, Mooshroom, Chicken, Villager.
Especially the chicken
To get this bug to occur, you close off a flat area with fences and when a baby turns into an adult in the corner, it will move outside of the fence.
This will not happen every time however it is very common