I know most of these stories are going to be IT or food service, so I'll chime in with mine to change it up.

TLDR: We caused some explosions on a transformer because someone didn't read test results.

I work for a power utility. One night, we were energizing a new transformer. It fed a single strip mall complex with a major grocery chain on it, so that's why it was at night, as we couldn't affect the service while they were open.

Anyways, we go to energize, close the primary switches and one of the lightning arrestors blows up. And I mean blows up, like an M80 just went off. Lit up the sky bright as day for a couple moments at 1 in the morning. The protection opened the switches and everybody is panicking, making sure nobody was hurt.

Well after everybody settled down, the arrestor was replaced, they decide to throw it in again. Switches come closed, and explosion #2 happens. A second arrestor blows spectacularly. I tried to convince the one supervisor on site to go for a third time, because why not, but he didn't want to do it again. Whatever.

A few days go by and we find out what the issue was. This transformer was supposed to be a 115kV to 13.2kV. Come to find out there was an internal tap selection that was set for 67kV for the primary, and not 115kV. So what was happening was the voltage was only being stepped down half as much as needed so there was like 28kV or so on the secondary instead of 13.2kV and that was over the lightning arrestors ratings, hence why they were blowing up. So the transformer had to have its oil drained, guys had to go inside it and physically rewire it to the correct ratio.

We had a third party company do the acceptance testing on this transformer, and our engineering department just saw all the green checkmarks but didn't pay attention to the values for the test results. Nobody expected to run into this because we don't have any of this type of transformer in our system, but that's certainly no excuse.

Moral of the story: read your acceptance test results carefully.