What I expected to happen was...:
Leash pops off when horse pulls away far enough and disappears from post, dropping the leash as item on the ground.

What actually happened was...:
Leash duplication bug and glitch. (see below for more info)

Steps to Reproduce:
1. Get a tamed and saddled horse, tie it to a fencepost.
2. Hop on horse and ride away.
3. Leash pops off but appears to still be on the fence, and when you try to attache again it bugs and pops off an extra leash as well as the leash you just tried putting on (so you use 2 leashes and get 3 back).