This was a long time ago, when I was working part time in my uni helpdesk. as part of the uni IT service, they offered ISP services at the dorms. It was cheap, and fast. This essentially allowed students living in the dorms to connect thier personal computers to the uni LAN. Then one day...
An ARP poison malware infected some of those computers. An arp poison attack is simple (look at ettercap) - it redirects network traffic via the affected computer, and adds malware to webtraffic to infect more computers. One of these on a network is bad enough, but when there more then one... traffic was redirected a lot. this caused the Dorm switches to collapse under the load. Fun times to work at the helpdesk...
The IT guys came up with a solution for this: they blocked the arp poision attacks at the firewall, and then disabled the switch port for the infected computer for 24 hours. so, when someone called with 'I have no internet!', we told them to bring us the computer, and installed an AV on it.
3-4 month the problem was cleared.