Domesticated mobs such as cows or sheep seem to be able to escape fenced areas.

The fences are complete, although they usually include a gate. If you look at the attached picture (apologies for the red tint) you can see a lamb in a fenced area with a cow. This lamb was "born" in the fenced area next to it and, somehow, managed to get into the cow enclosure through a fence. It is not just into adjacent pens that mobs can escape - they can escape through any side of the pen and often then just wander off.

This is a massively frustrating bug that I've now experienced on several different worlds. It effectively makes it very difficult, if not impossible, to develop a flock/herd since you keep losing mobs so often!