I had a job where I was doing both hardware and software. I was testing a new controller we were developing. The testing involved having the board exposed while under live power, so I could test voltage levels in various states of operation.

My brain failed me and I disconnected the leads from the 120VAC input before unplugging it from the outlet. Line meets neutral. Sparks fly. The room goes dark.

The servers were on UPSes, but the network switch between them wasn't. They were in the middle of some sort of critical transaction in some third-party application and it corrupted the database.

We had backups, but it was not a good day.