When there is a carpet on top of a fence, mobs can now get out. I don't know how, as I've never observed it, but since they are given no reason to jump, I think they're just walking through the fence, or clipping through it. I noticed that mobs are very easy to get into blocks as well. If i hit a chicken with an egg, it may clip into the ground and begin suffocating. These animals will not return to the fenced area at all, unless I coax them back through a hole in the fence, which renders all animal farms using this method useless. Attached are pictures of my chicken "enclosure" and the animals that have escaped. In it, there used to be far more, perhaps 10 or twenty times as many, all of whom are roaming free outside the fence. There were no mobs outside of the fences, and I know they didn't spawn, because blue dyed sheep are escaping as well.