Snapshot 13w06a, Snapshot 13w07a, Snapshot 13w09a, Snapshot 13w09b
Mobs with the old AI (Endermen, Zombie Pigmen, Spiders, Cave Spiders and Blazes to be specific) look the opposite way whilst attacking the player.
Steps to Reproduce:
1. Make a new Creative world.
2. Spawn one of the mobs listed above.
3. Hit the mob and fly above it.
4. Notice how its head movement is inverted.