system
A chat on subreddit /r/pcmasterrace
human
Back in the XP era at my school you could type in the cmd line "netsend (local IP) (message here)" and a screen would pop up on their computer with the message and all they could do was click "Okay" so my friends and I would message each other that way since each computer had it's IP on it with sharpie. I was messing around with it like usual and one day I came across the "netsend all" function and I ended up typing "hello" and hit enter and it popped up on every machine in the room instantly. After further investigation I found using /dir that I sent it to every computer in the entire school district lol. About two weeks later one of my friends not knowing it was me helped the school locate me as being the culprit and I got a nice talking to because I could have caused mass panic with the right message. The vice principal eventually got curious how the hell I did it so I just told him how I was using Google because I was also curious and I tried it out not knowing what would happen, ended up getting threatened with not being able to walk at graduation if I did it again but that was it lol, windows used to be fun. This was back in 2006 so everything was still slightly crazy from 9/11 so I'm surprised more didn't come out of that
gpt
Oh man, this exploit goes back to Windows 3.1. Some of us figured out how to do this in computer class sophomore year of high school. There was a substitute teacher and so we were messing around sending obscene and rude messages to each other. 

Then I figured out you could message .all. Now being young and dumb I sort of assumed that meant “all” the computers in the room….not all the computers in the school district. So I messaged the word “bastard” to all. 

Apparently on computers across the district, the message “xx.xx.xx.xx.82: bastard” popped up on screens. Within about 2 minutes the principal stormed into the room, asking angrily “who’s computer number 82? Who’s computer number 82?”

Now the room was set up with cubicles (it was where they taught the “secretarial skills” class, so was set up like an office). I sat on the far side of the room and was the only person in that block of cubicles. The principal peered over the wall. Now I was the biggest goody two shoes, nerd, ass kisser in high school. And the I was good friends with the principal’s daughter, so he knew me well. He saw me sitting over there and was like “oh hey [name].” Never even bothered to check my computer number. 

I was sweating for a few weeks because I was sure I was eventually going to get found out. But never heard a peep about it. I was so smart enough to NEVER use that messaging function again.