My worst ever was when I was in the Service as a newly trained teleprinter operator.  I was on weekend duty which meant working alone at a small switchboard in the exchange and also sending signals on the teleprinter.  When the NCO showed me around the weekend duty room to familiarise myself with the system, she must have thought it was a good idea to play a trick on the newbie and told me that if I couldn't get the signals out to the right number I could just dial the number on the teleprinter dial and send the signal to that one and it would be automatically forwarded to the right number.  I thought it was strange as in training the number on the dial was our own communications centre number but thought perhaps the system was different 'on circuit'.  Needless to say I had problems sending signals and, as directed by my superior, sent them all to the number on the dial.  When I went on duty on Monday morning I was called into the OIC Comms office, via the teleprinter room, whose floor was covered with reams and reams of teleprinter paper and punched tape!  Yes, I got told off and No, I didn't tell on the NCO - well, I was pretty green at the time.  Since then I have always trusted my instincts.