Timing: At work, we tend to send out direct mail or email to users driving them to various parts of our client's website. At times, this can get up to the hundreds of thousands. This is typical in our industry. After such a marketing "blast," our sites get a lot of traffic, and that is when this joke will be played out.
Scenario: On whatever day would receive the most traffic, I should intend to leave early that day. Around 4-5p (right after I leave), change the hosts file on a selected group of victims' desktops at work to point to the development server (or some other server for that fact). This other server would have much slander and evedince of being H8X0R3D with much severe deprecation to us, our clients and our clients' users.
Outcome: Adrenaline, screaming, sweating, wondering "where the hell is Mark" (as I won't be answering my phone), lots of good video footage of the victims freaking out, and, in general, a great time had by all (except for the victims).
I don't know why I stopped to write this now, as I'm supposed to have a client meeting in about 45mins up at work (I'm still at home).