MC-140813 revealed an easter egg that occurs when a player punches a panda near a villager; the villager produces angry particles, as if it were hurt. However, this is also the case when the villager has no AI; if a villager has no AI, it doesn't produce the particles when it (the villager) gets hurt, which is why I believe this is a bug.

How to reproduce:

1. Spawn a panda
2. Summon a villager with NoAI set to 1b.
3. Punch the panda