0.8.0 beta build 5, 0.12.1, 0.12.2, 0.12.3
i was herding chickens near my base which is surrounded in cobblestone walls.
when the chicken had to navigate its way around the wall, its head would appear through the other side.
once this happened, the chicken was unable to move away from the wall.
it could walk along the wall while it's head was stuck in it.
the player has to manually remove the chicken from the wall, and subsequently learn to avoid having the chicken clip through it.
steps to reproduce:
build a cobblestone wall
spawn a chicken
herd the chicken with seeds
position the player so that the wall is in between Steve and the chicken
allow the chicken to run into the wall in it's pursuit of the player
move the player away from the wall and attempt to get the chicken to follow in another direction
observe that the chicken will not move away from the wall
not sure if other mobs are affected by this.