During my first true IT role, rather than just being a guy good at IT in the office, I was tasked with the backup tape duties of the AS400 while the regular guy was off on holiday.
I was shown how to load the AS400 backup tapes into the tape hopper, and take the latest back up to be removed and taken to an offsite location, which was the security hut about 50 meters away. It looked simple enough. So on the Monday I took out the Sunday backup, and loaded it with a fresh set of seven tapes for the rest of the week. Nice, easy, job done.
However, I got in on Tuesday morning for the early support shift, and as soon as the clock had ticked over to 7:30am the helpdesk started getting a load of calls about the main ERP system being down. Soon about 350 people could not do any work. A quick call to the one of the AS400 guys sorted out the issue. It seems that I had loaded one of the tapes wrong and it had jammed. The backup process had failed, and the ERP system did not start up if the backup failed. The AS400 guy started the ERP system back up, people got back on with work. I checked the tapes; they looked OK, next day the same thing happened, Tape Jam the sequel, I had put it in wrong again. The AS400 person finally updated the start-up routines, so if the backup failed, the ERP would start. Thankfully, they never put the backup issue with me loading the tapes, and as the system had gone live within the last few weeks as part of a company takeover, they put it down to teething issues.