It's not 100% glitch free. Pigs can get stuck in the top leading to an essential mass culling at some point. The piston helps but doesn't always push all of them into the water.
I didn't do it in the video but replacing the middle/top block (above the half slab) with glass instead of a solid block helps prevent the pigs going mental when using the piston sometimes.
If you don't use it for ridiculous amounts of pigs it should be relatively hassle free.
Tbh I hadn't seen any real compact pig farms, If you know of any, I'd be interested to see them. All the smaller farms I have seen always seem to have a downside.
This one here for example, I always used it without a piston. After using it multiple times FPS would begin to drop and you'd need to kill 100+ pigs in the cell. I was messing about in creative and added the piston. It worked well, as you could see in the video, there certainly didn't seem to be any problems with it, but now it seems every time I use it pigs fly all over the place.
I'm using this now: http://www.minecraftforum.net/topic/2097903-minecraftisbrutal/ It has a longer footprint and a cobweb could be an issue for most but it really is 100% efficient, No glitches at all. The only issue would be how many it kills. Sometimes it will kill all of them if left on, sometimes it leaves 3. I'd just like it to be consistent one way or the other