Giving out the password to a privileged account
About 30 years ago, I was working on the sort of collaborative project where there was a librarian who signed out parcels of work to the techie types (myself included), and signed in the work packages when complete. The librarian's account was privileged to the extent that it could be used to change access rights to parcels of work in various states of completion.
The operating system was VMS.
On one occasion, I was working a weekend shift with not many people around, and the librarian came across to my desk, wearing a puzzled expression. She had tried to login to the system, and instead of the expected system response, the printer had jumped into life and had produced ... a single line of text. Or, to be more precise, more than one line of text, overprinted to appear as a single line. She showed me this page, thinking that - as the sole techie present at the time - I might be able to figure out what was going on.
Disentangling the overprinted text, I realised that there were in fact two lines (as I mentioned, overprinted). And they could indeed be disentangled character by character, and I could make intelligent guesses as to which characters belonged to which line. At which point ...
[Myself:] "You do realise what you have just done?"
[Myself:] "You have just given me your password."
[Librarian:] "You know my password?"
[Myself:] "I do now".
I then showed her how the two lines of overprinted text could be separated into a plausible userid for a librarian, overprinted with other characters which could reasonably be a password. So now there were two pieces of paper with the password: the original print and the piece of scrap I had used for the demonstration.
[Myself:] "I suggest you do two things: change the password, and destroy those two pieces of paper".
I still don't know how she accidentally subverted the system so as to get it to print out her userid and password. It never happened again.