I had started a 1.5 world with friends (over lan) updated to the newest snapshot.
I had 5 baby chickens (to be honest i'm not sure if it's just baby chickens, or baby mobs or all mobs in the game where this occurs).
Upon logging in I noticed one escaped, four remain inside the wooden gate (and they even looked like they could walk right through the gate.
Stopped playing.

Hours later log back in singleplayer another chicken out of the fence.