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R.I.P. Yapper. We love you!

@Ben The issue you are referring was recently fixed: MC-2416

Happens with skellies in single player hardcore mode not modded at all. You can replicate by stacking hundred of skellies from a spawner in a 1x1 hole. The mob will visually seem to be passing thru a solid wall and he will wander around but he is still trapped. He can't hit you or vice versa - he is like a ghost. After a few seconds (probably a chunk update) the mob freeze and slide directly where he is supposed to be. It is only a visual glitch.

To my experience, I never saw a mob (zombie, skelly, creeper, spider) penetrate a wall/fence except for a small glitch MC-2713. The skeleton will visually seem to be passing thru a solid wall and he will wander around but he is still trapped. He can't hit you or vice versa - he is like a ghost. After a few seconds (probably a chunk update) the mob freeze and slide directly where he is supposed to be. It happen when you stack hundred of skellies in a 1x1 hole in single player.

Since 1.4.x, I noticed that animals have the ability to wander inside solid blocks. To reproduce, dig a 3x3x3 hole and place a cow inside and push it. The cow will go inside the block and turn black when he reach the middle point. When you release him, he gets out of the block slowly (spring effect). If another animal push hard, it can even make the animal suffocate or die. The reason I say that they escape to the north is because of multiple witnessing of this bug. I built a reserve bin that is divided in cells. All animals are problematic for me. When they are in a fenced farm, they wander randomly, but when they reach the west limit of the farm, they push, push, push to the north and that is where death or escape occurs (I always see the animal in a specific bin, if I close the bin, I see pieces of meat in from of the cell. If it is a fence or solid block, the animal pass right thru. Go figure. If you have a thick wall, the animal suffocate and die. Of course, if you have an over-crowded farm, they may escape everywhere. I think the best way to fix this bug is to completely prevent animals from passing thru block and fences by reducing the penetration distance of animals of exactly one block (except for chickens, since they are smaller) or increase the spring effect to surpass any other pushing effects from animals, players and mobs when the penetration reach a gap of around half a block (still half of a block too much but better than a full block). The facet of the block should be the penetration limit, that's my opinion. Do you guys remember old versions of Minecraft? Everything was perfect in term of physical collisions. Now, it is very berserk.

I am sick and tired of hunting down those escaping animals. I am making engineering devices to trap and bring the animals back in the farm, it is spacious, noisy and ugly looking. The best fix I found is to build a reserve bin all around the farm but I am having difficulties to bring the animals back in without adding catatonic piston and water noises. Please, if someone finds a way to circumvent this annoying bug, could you post your ideas? Thanks.

Animals escape to the north. If you want to have an example, look at the screenshots with sheeps. That is exactly where they escape.

I love all the new features but some old bugs never get fixed.
Dinnerbone! Please fix this bug, please.