What happens:
I've got a sheep farm (screen). Sometimes instead of one of the sheep I can see a wool and raw mutton or sheep is outside the building. Nevermind if it's a block-wall or a fence. It happens even when server is all the time on. Second screen shows a light grey sheep randomly appeared in orange-wooled sheep pasture.
EDIT: And the 3rd screen showing what I found when coming back from mining.

How to reproduce:
1. Make a sheep pasture made of fence or solid block
2. Spawn many sheeps to speed up the effect
3. Wait and see:

• sheep outside the pasture
• wool and a mutton instead of sheep