Jumping backwards on a horse causes suffocation.
When riding a horse and jumping backwards, I would expect not to take damage.
However, when doing this, you do take damage. (Don't know if this is intended or not.)
Steps to Reproduce:
1. First, mount a horse and walk up to a solid wall.
2. Face your back to the wall and push up against it.
3. Jump. The higher you jump, the more suffocation damage you take.