When a player dies near a shulker twice [maybe more, maybe less] and respawns near the shucker, the shucker's model is hidden. Nothing can attack that shulker. When other shulkers projectiles run into the invisible one, they stay frozen in place. The invisible shucker continues to attack for a bit but then stops firing. Upon logging off and back on the world, the invisible shulker's model is restored and it continues with normal behavior.

Shulker projectiles still appear stuck in the head of the mob upon relogging.

Tested only in Minecraft 15w34a.