When I went into 3rd Person mode by pressing F5 twice while riding a horse, I noticed that my head rendered the wrong direction.

I expected for my head to follow the cursor.

My head would flip around and look where I did not want it to.

Steps to reproduce:
1. Find or summon a horse in Snapshot 15w31a.
2. Tame the horse.
3. Press F5 or whatever your 3rd-Person key is set to twice.
4. Move the cursor and notice that the head flips the wrong direction.

I tried this with both an invisible horse and a visible horse to be sure. Tested on 7/29/2015.