A computer admin at the school I attended bought a new proxy server to stop our file sharing. It was supposed to block "bad" Web sites in addition to filtering out some of the ports that P2P programs use. Trouble was, all we had to do was use a service such as Proxify to get around it.
The whole student Internet service was set up poorly. It was on the same connection as the school's computers in the labs, so anytime students started to browse heavily in the dorms or on the campus wireless, all the computers suffered massive slowdown in bandwidth.
The admin was supposed to tell everyone to change their TCP/IP settings so they'd input the IP Address of the proxy server. He left everyone in the dark about this, though, relying on word of mouth to get the job done. (This is the same guy who freaked out when he saw us running Linux on our laptops -- he claimed Linux was only for hackers. He urged us to stay away from it and just run Windows XP.)
Normally, the IP address for your computer is assigned by the proxy; you do not set it yourself. A female friend of mine, not to let an opportunity for mischief escape her, got everyone to assign themselves their own IP address, which was that of the proxy's. So when anyone tried to get on the Internet, they were actually directed to the computers, instead of the actual proxy, which shut everything down.
Not content with that one episode, later that week my friend "accidentally" used some command-prompt tinkering to change the servers' MAC address when the admin was trying to figure out how to change his. This escapade had the Internet down for another week as the computer admin struggled to figure out what was wrong.
Maybe it was mean to perpetrate this kind of fun, but we really were mystified by the guy's continued employment at the school. I was once in his office -- it was littered with "For Dummies" and other self-taught books. The school faculty believes he's doing a great job, but the students know what's truly going on. Until the school ends up with a teacher who is actually knowledgeable about computers, no one will know how inept this IT guy really is. And students will just continue to make his life a living hell.