It was fun to watch my entire high school (~1200 people) freak out when I ran "net send * Big brother is watching you..." on what I found to be an insecure computer in my high school's library. Every single computer in the building displayed the pop up message. The town's IT director even showed up to figure out what happened.

I was caught, but they were more happy it wasn't a hacker, and that I discovered that the IT firm the town hired totally botched properly implementing network security, so I was let off the hook.