Our web access went over secured proxy servers, which determined via our windows login which services to allow or disallow us. As our deputy rector considered himself an open IT advocate, he ensured all browsers we're installed on all machines, including Opera. Back then Opera couldn't use the Windows login to auth with the proxy server, which he "fixed" by using a pre-configured user which just had student rights.
As i'd never use opera i didn't notice it until one of my teachers tried to block me from using any webmail client as a way to transfer files to the school pc.
Trying different things i got the tip from a classmate to start opera. I was struck by a preentered login box which i was told to just hit ok. I got curious and inspected the fields. The loginname had admin in it, so i thought the password might be used elsewhere...
Well, i've seen our deputy rector on our central win 2003 server via windows remote desktop before, so i connected to it, using the user and pass from opera and logged in to an open session on the server which stored all critical school data, including grades of all students attending that school and the transfer tool to send the grades to the statewide server which would make the grades final.
As it was so ridiculously easy i got scared that someone from my class would do the same and then get into deeper trouble for actually trying to change grades or something, i immediately went to our deputy rector and told him all about it.
He banned me from using school pc's for the rest of the year and afterwards all teachers were told to keep their eyes on what i was doing on the school pcs...