Was the early 2000s, but in my high school library I did a

   NET SEND comp1231 i know where you live

to mess with my friend who was sitting right next to me. Then the hacker in me decided to see if asterisk worked (of course, without changing the message)

   NET SEND * i know where you live

(Hint: it works). Everyone in the school who had a computer on got the message. I caught hell for that one, had to write a letter of apology to a bunch of school staff who didn't "feel safe" after receiving the message. Never got access to my account after that either, so I always had to get my friends to log me in.

I didn't break the network...but hey, I helped expose a vulnerability, right? That's something.