When riding on a horse (or any of its variations) and shooting his back with a bow (shoot directly down or directly up so it lands on him) the arrow collision freaks out.
The arrows won't hit the horse nor pass through him and land on the ground, they jump around on its back.

How to reproduce:
1) Spawn a horse / Find a horse
2) Get on the horse (taming is not a must, but helps staying on him)
3) Shoot directly down or directly up with a bow so the arrow lands on his back
4) Watch them bounce
5a) If the horse moves away the arrows fall on the ground.
5b) If you get off the horse while the arrows are jumping on him they will land on him and damage him (this bug occurs only with a rider on the horse)