"Last September, the loose affiliation of
programmers who monitor the Gnutella file-trading network
noticed something strange. The network, a popular hub for
MP3 traders, seemed to be suffering a kind of
denial-of-service attack, with some people reporting that
their machines were inundated with requests for content.
Though the attack seemed small, the particular design of
Gnutella -- a "decentralized peer-to-peer network," in
which each computer routes network traffic -- amplified its
effects, causing the whole network to clog.
But when the developers got to the bottom of the problem, it turned out that there was no malicious attack --
it was just selfish code." More at Salon.com.