Basically this is what I have:
I wanted to start a sheep, cow, chicken and pig farm arround my house and built some fences to keep them in. It seems these fences do not work due to the mobs being able to just get through the block. I tried making a solid block layer outside the fence but mobs still were able to glitch out. This issue has persist for many versions and I've realized it doesn't take too many mobs to do the trick (Had 3 pigs in a 12x10 area and 1 got out).

The only fix I could find is to put steps on top of the fences, if not they would get out.