hmm, i have only done something like that once, i did to a friend (yes we are still friends)
we did some backup over network (using knoppix CD on his) to my "server", that is my 350Mhz laptop with a external 200G USB hdd.
I started ssh on his computer. then i started the sftp session from nine. after a while i got bored and i ssh'd into his computer and did cat /dev/urandom >/dev/dsp.
WOW he jumped from his chair and started screaming that i had made him use linux and that Linux had broken his computer... then he saw me Smiling and trying not to laugh, something must have become obvious as he started calling me all sort of names