In my cow farm, baby cows and normal cows keep walking through the fence! This just started when I updated to snapshot 13w09c, it was ok in 1.4.7 though. This seems to happen mostly when you are breeding the cows.In the screenshot below, you can see 2 baby cows from inside the fence walking through it to get to me (because of wheat in my hand. All the cows in the picture were originally in the fence. It also happens to adult cows, but more commonly baby cows.