Passive Mobs escape their pens

This bug has been around since fences were introduced I think. The fact that you can be standing partway onto the block the fence is on is part of the problem. If you ever stand around and watch groups of animals in 1 pen push each other you'll notice they go a little bit through the fence but then snap back to where they should be. I'm pretty sure when you save and quit you run the chance of them pushing through without the snap back. The snap back was 1 second too late to happen before the save, and is forgotten completely on reload. I just use glass block walls instead of fences. The glass blocks take up 100% of the block they're on and so don't have the problem.