Just started a new test world and found a zombie spawner. Layed down torches and despite full lighting was slaughtered by zombies spawning in full light conditions. a number were spawning in the floor and dying of suffocation.
Seed: -3125726014909501488
375, 16, 284
think I found a work around for this. I dig down one block and then have all the edges fenced. since starting this configuration have notlost any further mobs
just to be more clear the fence in down one block from the surrounding area.
seeing it on 13w09 and 13w10. Mostly my chickens are escaping. Oddly somehow a cow escaped, but I ended up with an extra cow. This is in an underground fortress so it wasn't just a cow wandering in. I always keep 8 cows and 6 chickens, so works with small numbers of animals as well.
lots of torches - still spawning (in the floor - but they pop up quick and only take a little damage)