I work at small companies and in addition to being an electronics engineer I'm usually IT dogsbody as well.
My worst day in IT was when I managed to take down all power to a server room while attempting to show off some fancy new gear.
We had an electrician in to do something that required all the building power to be off for about 20mins. I had also recently got management to buy a lovely second-hand behemoth of a UPS with modular and redundant everything.
So it was solely providing A+B feeds to everything instead of the two very overtaxed tower UPS we had before.
The electrician asked how long I needed to get everything shut down and I proudly told him we didn't need to do that as the fancy UPS had reserves for an hour. I then reached into the rear power panel to open the breaker for the mains incomer and got...the EPO (Emergency Power Off) breaker instead.
The noise of a room full of servers and lots of networking gear spinning down from hurricane to silence is possibly the worst noise in the universe.
I told the electrician I had to go but that he could start work and then did the adult thing of telling the staff that unfortunately a power cable had been found to be defective while doing essential maintenance but that I would have it fixed very soon.
With incredible luck all the gear came back up cleanly with no data loss. Not my finest hour though