Mobs Pass hrough Cobblestone Wall

I had this problem with the Cobblestone Walls lately. My farmlands have those Walls to limit the Cow Cells, Chicken Cells, etc... and sometimes I see the mobs out of their place (Cows in Chicken Cells, Sheep outside of their Cells), even though I hadn't opened any Fence Gates/Doors and there aren't any "holes" in the Walls. Any help is appreciated!