What I expected to happen was...:
The Lead would Connect to the fence post from the animal

What actually happened was...:
The Animal was Connected to the Fence post, albeit the texture of the lead stretched out extremely far in a south east direction and begins jerking around extremely fast.

Steps to Reproduce:
1. Attach a lead to any leadable mob in minecraft
2. Right click on a fence post with an empty hand
3. Observe as the lead(s) fly off in a South-East direction

Note: Tested with multiple different mobs and multiple different Fence Posts both in Survival and Creative mode.