I was 21 and at a multinational IT consulting firm. Target was one of our clients, and at that time, used the IBM dumb terminals for communication. The 'Message Send' command was used similar to IM now-a-days, but if you do not provide an id, it sent the message to ALL users (which few people knew about).
One Friday I sent, what I thought was a joke - "To commemorate Blaise Pascal's birthday, all your computers will be run at half-speed today".
I thought it sent the message to all IT folks, but what I did not realize was that the message was sent to ALL users - retail stores, Target business users, management teams etc.
And that Friday turned out to be the 13th. So Target security thought it was a virus attack, and traced the origin of the message to, naturally, my cubicle. I was called into work by my CTO at 2 AM, and I, with a pale face, explained how it was supposed to be a harmless prank. Then the CTO and with the CEO on the phone crafted an apology letter to the Target CEO/CIO with me right there. I was pretty sure I was fired, but I wasn't.
On the plus side I became famous and made lot of new friends in the company :). And Target implemented a change to the IBM send message to prevent this.