I remember in high school poking around a network drive until I found an executable with the name "SEND" in the name. I had a sense that it would send some kind of message somewhere, but I honestly didn't know where or to how many people. I was quite surprised when all the screens in our computer lab froze and, five seconds later, my message appeared on all of them. (I later learned that my message appeared on every desktop screen in the school!)
I'm not sure exactly how they found me out, but I was called into the IT admin's office a couple of days later. She was furious with me. I told her the truth. I didn't know what exactly would happen when I ran that command, but she didn't buy it. Fortunately, nothing ended up happening after that.
I've wondered to this day what exactly they could have done to me if they decided to press whatever legal authority they might have had to its fullest extent. I was never told "don't go to Z:\" or "don't run any program other than those on this list." Even after I was found out, I wasn't ever explicitly told that my actions constituted unauthorized access.
It was a different, perhaps more innocent (or ignorant) time back then. How much have things changed now?
Wasn't a regular MS user, but we were in a computer training lab at a company for "computer day" field trip. Was bored during instructions, so naturally I logged in, found "net send", and sent a few crank messages to classmates using * as destination. Everyone, including the instructor, got a good laugh.
Approached later in day by corporate IT. Apparently the lab had poor routing rules, no firewalls, and sat on the main Corp network. My messages were received on 25,000 terminals.
Thankfully, they recognized this as (a) harmless, and (b) their own lax failure. No adverse outcome.