We had a Cyber mainframe at uni, and the process for signing up to it was to use your student number, date of birth and full name to authenticate. Of course the CS dept employee (an introverted grumpy bastard - very BOFH) left the data file readable in his home directory briefly so we scored a copy quick smart and had unlimited accounts, (cpu quotas, disk space etc) for years.
The dodgiest part was one day I decided to print it out to have a hard copy, just in case. At the time we had huge fast drum printers and an Operators room where you went to collect your print jobs. That day the employee in question was staffing the print out desk and I watched with dreaded fascination as he went down the stack of printouts (1/2 meter tall usually), flicking back the cover sheet with our student numbers and fanning down to the next one to seperate out each job. My all-students-in-the-uni print job was about 20cm high, and to this day I don't know how he didn't ken to the fact that it was that particular file from his home directory I'd just printed. It was a phenomenal waste of paper which you'd at least glance at the contents to see why someone had 800 pages of output.
Getting away with that in that fashion has always been one of the funniest memories from that institution, and it leveraged a lot of shenanigans for years on end. The students had the keys to the kingdom and we damn well used them.