Minecraft wiki entry for Wither_Skeleton, associated text: Wither skeletons can't fit through the gap under the staircase of the nether wart room in fortresses.
If a Wither Skeleton is summoned in via commands to hold a bow, it shoots fire arrows, even if the bow doesn't have the Flame enchantment.
The wither skeleton's first, second, third, and fourth hurt sounds are the regular skeleton's first, second, third, and fourth death sounds respectively but heavily edited.