I don't think these guys dammaged anything, but FBI did not appriciate their humor ....full article here
Fluffi Bunni embarrassed leading Internet security organizations by breaking into their own computers and replacing Web pages with a message that "Fluffi Bunni ownz you" and a digital photograph of a pink rabbit at a keyboard. The attacks, which began in June 2000, lasted about 18 months, then stopped mysteriously and created one of the Internet's most significant hacker whodunits in years
[...]
The guy was playing a game of `gotcha.' He wanted to prove that even firms that specialize in security can be hacked," said Mark Rasch, chief security counsel for Solutionary Inc. and a former Justice Department cybercrime prosecutor. "It's like someone who robs banks to prove that banks can be robbed.