When adult chickens are standing on a half-slab (bottom one) with water above their heads, they suffocate instead of swim up.

Steps to reproduce:
1. Place glass 3x3 and remove the top, middle and bottom center blocks. (This is to contain the chicken and to view the problem.)
2.Place a bottom-facing slab on the bottom of the hole you made.
3. Spawn chicken on top of slab. (Important: must be an adult chicken.)
4. Place water the block above the slab.

What happens: Chicken suffocates.

What should happen: Chicken swims to avoid suffocation.

This happens because the chicken is both in and out of the water block. Except for water mobs, mobs are supposed to swim to the top of any water block they are either in or partially in to avoid suffocation.