Upon loading a world that has multiple entities confined in a small space or up against a wall while affected by a water current especially a 1x1 area, the entities will phase through the terrain, escaping captivity and creating a huge mess. They do not seem to fall through floors, but will go straight through multiple-block-thick walls. The force that propels them seems to be mob overlap repulsion, but it can also occur with water, allowing this bug to affect a single mob with nothing (but walls) nearby.
If I had to guess, the issue is that mobs begin moving before terrain physics are loaded properly - when you load into a world, you can catch a solid five seconds in which chickens flap their wings as if they were airborne. For some reason, this does not affect the player at all.
Whoops, forgot to mention this happened on my Default world...