Schoolboy stuff really...
When I was Uni, Netscape hadn't long been out and allowed users to send email from whatever address they cared to enter. One of my mates spoofed the address of some cabinet minister and sent it to another mate, telling him he was a cunt. The addressee (obviously knowing it was a joke) replied with some rather defamatory remarks thinking it would go back to the chap who sent it. However the return address was the MP's so instead it was delivered back to him.

About a week later there was a reply stating that the email definately did not originate from him and would he like the Police to investigate.

Also sent loads to my mates pretending to be Customs and Excise. I'd state something along the lines of, "we realise that you have illegally been accessing pornography and could be prossecuted under current UK law. However if you wish to contest this in a court of law, please feel free to do so." Most discarded it, but one guy was very upset and eventually got round to telling me all about it. I told him to get back to them and take it to court as the case would never stand up. Needless to say he didn't...

Later I worked for a company where all the computers had the same local admin password. This meant that we could copy files to each copy as we pleased, including placing them in the user's start up directory (so that it ran when they booted.)

I created a simple program that could be remotely controlled to pop up windows messages whenever I wanted. I waited for choice moments, such as just after the systems guys had installed service packs, then would bombard the hapless user with error messges such as "Windows Error! User is a gimp." or "Please stop surfing porn, user may go blind."
(Swirly, Tue 10 May 2005, 17:21, Reply)