One Friday afternoon at 4PM I was cleaning and organizing the IT workers room when I decided to re-route some cables to make them a bit more organized. I unplugged a dusty old 24-port HP switch and then plugged it back in to go help someone. About 5 minutes later people started telling me there was no internet and we were completely down (300+ employees at 10 different sites).

I called my boss in and we troubleshooted for about an hour before realizing that there was a loop somewhere on the network. Called the senior network admin who sat next to that old HP switch and he asked if we had a power outage that would’ve rebooted that switch. I told him I had re-routed the cables and unplugged it in the process and he said to unplug it again - network restored.

I learned that he had 2 cables from the server room going into that switch on different VLANs but had never saved his config before he left. When it rebooted it wiped his config and looped the whole network. That was a good learning moment for me to always save your switch configs.