Sometimes with villagers, zombies, or other animals, they are capable of glitching through things like fences and doors. It was very frustrating since I was trying to make a villager city.
I expected the villagers and animals to just bump into them like solid blocks.
Instead, they sometimes "glided" through the blocks. So I had a bunch of pigs and zombies running around my city causing havoc.
Steps to Reproduce:
1. Spawn lots of pigs or other kinds of animals
2. Keep them in a fenced enclosure
3. Occasionally you might see them just float through the fence as if it were air.