Yup, I did the same. A contractor replaced all our Classic Macs with Windows 9x boxes and never taught any of the staff how to secure them.
We were using winpopup to send messages to each other, when I worked out how to send them to the entire subnet (or domain - I forget). Three months later I was hauled in front of the Deputy Rector to explain how I managed to 'write my name' on the admin office server. It turned out that they never turned on the monitor connected to the print server until the printer stopped working and they jumped to blaming me.
It took some really fast thinking to wriggle out of that one - I was warned 'he'd be watching me' from then on though.
The stories seem to be going backwards in time, so I'll chain my 80's version of this. I wrote a fake login prompt on our VAX that looked exactly like the normal login prompt on the VT100 terminals. It would record the userid and password in a log file, give the "wrong password" message, then exit/logout so the real prompt was there for their second try.
I'd launch that on a few terminals on my way out of the lab.
Never did anything notable with it, but didn't get caught either.
I believe this sort of thing is the primary reason for needing to ctl-alt-del to login to windows. Though I'll bet most people wouldn't think much of a (fake) login screen just being there ready to go...they'd probably try to login anyway.
Yeah, but not quite. When you sent the message it used your network I’d as for the ‘from’ field. I was even that dorky that the message I used to prove it worked was “Hello, World!”. Try explaining geek memes to high school teachers in the late 90’s.
Thankfully the time i took down the high school's network using a looping bat file that opened looping NetSend commands to the entire domain the teacher of the CCNA class I was in thought it was hilarious and more of a shame on him than a shame on me.
I used NET SEND to do something similar, a little later on. In my previous experiments, this caused a message to appear on all laptops on the local network, so when it didn't, I started sending... less than savory messages. Screaming into the void, as it were.
Unfortunately, the messages were appearing... on the domain controller.
I think I got a detention and a couple weeks ban from using the computers. Fair enough.
As an idiot I used my account and immediately got suspended.