what you can do is pull out 2 network cables from 2 computers discard on of the network cables then plug the other into the socket of the other one so they loop around.
If the network hasnt been set up with top of the line hardware, this creates a "broadcast storm". The DHCP server will send out a broadcast, it will then loop back and be recived by the server again to send out another broadcast. On standard typical run of the mill hardware this will cause Massive problems and slow down all the computers and on windows XP machines for some reason it maxes out the CPU as well!
How exciting. more information http://en.wikipedia.org/wiki/Broadcast_storm
We did this a few times to our school to get out of homework n stuff;) sometimes it would take them hours to figure out which cable was crossed..
so if you wanna be super cunning, you could snip up a lil cat 5 cable and put it between to ports where you could never find it or something evil like that..