After I attached a lead to a cow, then attached the cow to a fence post, I proceeded to kill the cow. Upon killing the cow, all but part of the lead that was on the fence itself disappeared. I right clicked the fence post to get my lead back, but all it did was cause the lead to disappear.

I rather expected to get my lead back, considering if you tied a cow to a fence in real life, then killed it, the rope would simply drop to the ground, and you'd only have to untie it from the fence to get it back.

Easiest way to reproduce is to attach a lead to a cow, attach said cow to a fence post, kill the cow, then right click on the part of the lead left on the fence. I tested this twice upon discovery in creative mode, and the results are consistent. No lead dropped.