Mobs can escape enclosures and sometimes suffocate through walls.

I have an enclosure with sheep in it. It is fully enclosed and is two fence blocks high. Despite this, my sheep still manage to get out. So far, in about two weeks of playing, this has happened three or four times. I have never seen the sheep get out, nor can I reproduce it, but I'll come back from another part of my world and one of my sheep is roaming around on the outside of the pen (and I know it's my sheep because they're dyed blue).

I'm assuming it has something to do with that section/chunk of the world being loaded and unloaded, and it can also happen if the world is closed and reopened. I thought maybe when that chunk unloads sometimes the sheep have their heads sticking through the fence, so when it reloads it doesn't know which side of the fence the sheep is on.