Sheep, and possible other mobs, still glitch through fences

Even though it was patched in 1.6.2, my sheep still escape from both wooden and stone fences when reloading the chunk they were in. There are about 6 sheep in a 7 by 6 block area, so none are being pushed. When I leave (the chunk unloads) and come back, or when I re-log, they occasionally glitch trough the fences.