Skeletons are sometimes unresponsive.

Sometimes I find skeletons that are unresponsive. I try to hit them but they take no damage. Even during the day they don't burn in the sun. This may be applicable to other mobs. Arrows also bounce off skeleton and are shot back at the player.

What I expected to happen was...:
Mob should be responsive and receive damage and attack player.

What actually happened was...:
The mob was unresponsive and sat there doing nothing.

Steps to Reproduce:
I don't know entirely how this can be reproduced. It probably happens when I change the difficulty to despawn mobs then set it back to normal.