Out of boredom, just kind of want to know what you've seen.

Worst one I've ever seen were the Nazis in Nexuiz. They used to VPN, clone admin names, fake as admins when the real ones were gone, then drive all the players out. They had VPNs, so nobody could actually get rid of them by banning. They would keep doing this until the server owners gave up and took it down.
In the end, a server was either taken over, or taken off the list.