1990, freshman in college, Pascal class on AT&T Unix SVR3 3B2 cluster named "earth", "wind", and "fire". We'd just learned our way around "vi" and how to "uvapc" our Pascal source into a.out.
I discovered anonymous ftp, and "make", and I quickly rose to become the gaming king of Pascal class. I had ularn, nethack, megs and megs of games crammed in my no-quota $HOME, and I'd opened permissions for everyone else to access and play them. I often chit-chatted with a classmate or two over "write" or "ytalk".
Not content to merely play games, I became mischievous with the system and its inner workings. I created a .profile and a .plan, the latter of which used VT100 cursor escape sequences to self-modify the screen, such as changing my $HOME to "/" and I also made a boastful comment about having root access.
It was all in good fun, and then came the day that I discovered /etc/passwd and I attempted to "su" to every single system account I found listed. I mean some of their passwords were just "*" so they must've been wide open!!1
I soon received ominous, chilling email from the unseen sysadmin of the whole cluster. He described to me everything I'd done up to this point, and he informed me that saying I have root access is like telling airport officials that I have a bomb. That point definitely drove it home to me as a young and dumb hacker.
So, for the rest of my short college career, while I did some silly "extra-curricular" things with my own compute resources, I was careful to not try and break system security, or even say that I had, for fear of the wrath of the unseen sysadmin.