Easily replicable; place fence in a small 1 block by 4 block rectangle (the interior). Punch the cow to make it go crazy (It works with all animals, but the bug is more easily replicable due to it's speed and size), and then place a fence EXACTLY WHEN THE COWS ENTER THE BLOCK (Maybe even a second or two before, it needs to be really precise otherwise it doesn't work, and sometime it doesn't for no reason). If you get the timing right, the fences will extend as if there was a fence in the middle even if there isn't (A mob was in the way so it wasn't built)

It can take quite a few time (in plus of needing precision and good reflexes) to recreate the bug, but it's actually not that hard to accidentally get

Not sure if intended, works both in survival in creative. Works in singleplayer, didn't test in multiplayer