Some mobs can trespassing fences when there are a lot of mobs in a corral or a close space. When they´re not trespassing blocks of any kind, they literallly can "poke out" out of the fence and sometimes, blocks.
Steps to Reproduce:
1. Put a lot of mobs in a fence or a space made of 4 walls (sorry my english isn't good at all).
2. if you want, you can fill mobs untill it's enough for fill. (some of them can show their head or body slightly out trespassing the fence. I recommend use cows, sheep or pigs.)
3. Now fill a lot, but a lot of mobs untill the fence or the hole is exceeding the capacity of the corral, than mobs could get out of the fence inflicted by a bug.
Please see the attached video and the pics below.