Hitbox of a wither skeleton is shorter than Wither skeleton itself. It's most noticable when using bow.
How to reproduce:
1) Find a Wither skeleton
2) Take a bow and aim at upper half of Wither skeleton's head (take distance into account of course)
3) Shoot -> arrow will fly through the upper half of it's head
Tested on 1.4.4 server with 1.4.5 client with enchanted bow (Power IV, Fire I, Knockback II).
Jens Bergensten
You can easily see this by pressing f3+b