I have a fairly simmilar Problem and i have made a seperate bug report, but i am commenting on this so maybe it may be seen. My problem in 1.6.2 is that Mobs (in particualr animals such as sheep/cows), when fenced in an area with fence, glitch and pass through the fence. they do so when simply attracted by a piece of wheat. i have tested it on creative and i have shown that it takes about betweet 5 and 30 seconds for sheep (in particular) to be attracted by on a straight wall of fencing, to pass through the fence. No chunk loading, or even re-logging; the mobs just go through the fence.
I have a fairly simmilar Problem and i have made a seperate bug report, but i am commenting on this so maybe it may be seen.
My problem in 1.6.2 is that Mobs (in particualr animals such as sheep/cows), when fenced in an area with fence, glitch and pass through the fence. they do so when simply attracted by a piece of wheat. i have tested it on creative and i have shown that it takes about betweet 5 and 30 seconds for sheep (in particular) to be attracted by on a straight wall of fencing, to pass through the fence. No chunk loading, or even re-logging; the mobs just go through the fence.