Hitting a cow while it's on a lead, then setting the cow loose results in the cow permanently running at the speed and style it normally runs when hit.

Honestly, I rather expected it to stop running like it normally does after a hit once you remove the lead. While the fact that it doesn't stop running at all makes a kind of sick sort of sense, I seriously doubt you all were aiming to add 'abused cow' to your mob list.

It's almost absurdly easy to reproduce this, and while I haven't tested it, it wouldn't surprise me if this affected all the other animals that are set to run at fast speed in a chaotic pattern after being hit. Just attach the animal to the lead, hit it, set it loose, and watch it run like a chicken with it's head cut off... only it'll never stop.