Mobs glitching through fences

Fenced in cows have been getting loose constantly, I'm on singleplayer and 100% sure they can not get out only about 10 entities in a 7x5 area and after leaving and coming back at least 2 cows are out. Also villagers in a breeding cell consistently are getting loose through the fenced in area. Both pens are 1 post high. Was playing on 13w09c and there was no problems updated and they started getting loose consistently.