In high school - around 1972 - students in the rather new "Computer Science" could use the Board of Education's IBM 1103 for projects by submitting card decks, which were run and returned with printouts of the results. I went to the local IBM office and bought the 1103 system manual.
For some forgotten project, I wanted to alert the operator to do something... adjust the printer or whatever, and used a "//" command in the job control cards required in each deck to print out a message to the operator on the Selectric system terminal. Almost nothing ever typed out on that so I figured it would get attention.
Apparently that command redirected all printing to the terminal rather than the 1403 line printer.
They had to reload the OS from punch cards, several full boxes. This system printed the pay cheques for the entire board, dozens of schools and many hundreds of staff. Not very practical on a Selectric.
Needless to say future student projects were less welcome. I was among a select group of "bright" students who had unsupervised physical access to the computer room after hours... that access program was promptly cancelled and I was specifically banned for the room for life. I was, for a while, none too popular among the other students who had access. Until I managed to get us after hours access to a commercial PDP10 timesharing system - we didn't mess with that one.