My school hacking story: 7th grade, springtime, ~1998. The district used software that ran on login and populated your desktop/start menu and permissions. This was a mixed network of windows 98 and XP for all the newer computers. I found a bug where if you corrupted your own user profile folder, windows would load a temporary one after reboot and not apply all the restrictions, giving access to explorer. You could also get access to explorer by going through the f1 help menu in a couple of different programs.

Promptly used explorer to navigate to my english teachers computer via the hidden c$ share, and delete the executable from the program files folder. Next time she logged in, BOOM nothing. no start menu, no desktop, no permissions. The admins had an incredibly consistent and predictable naming scheme, and my idiot "friends" I shared the vulnerability with promptly used this to nuke like 3 labs and a bunch of teachers computers.

Fast forward 1 month, we all got pulled out of PE by a cop and sentenced to 1-3 weeks of community service.

* I abused that profile bug to work exclusively out of portable firefox on a usb drive instead of being tied to internet explorer 6 and 7, which allowed me to bypass proxy settings and get access to gmail and read slashdot/ign/halo.bungie.org during school hours! Those were the days.