You can completely crash a server with the jockey brush.

Steps to replicate:

You use the jockey brush on someone.
That someone uses the jockey brush on you.

What happens?

Player movement and other things will come to a halt, while chat will keep working.
After a minute or little more the server will crash, displaying a read error, and it will not continue to work unless you stop or reset it. The crash also doesn't get picked up by Bukkit Remote Toolkit, so it never gets restarted.