I was a newbie on my first-ever team assignment to take out a company's old networking equipment and upgrade it to new.
We arrived and got started; everything went fine. After four hours we took a lunch break. Someone who worked in the building (for our purposes, "Bob") came to us and asked if we could take a quick look at his computer; he couldn't get the printer to work and the company's only IT admin was out at lunch. Being the overconfident newbie, I said, "Absolutely!"
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It was a small office, and the printer was on the opposite side from the desk. I followed the USB cable from the printer to the computer, and the connections felt secure. I thought maybe there was communication issue, so I checked the software and tried a few test printouts. It had the printer listed, but wasn't doing anything at all.
I couldn't figure out why it was listed but not working. I happened to catch that the printer information was somehow saved as a permanent listing, and due to some sort of software issue (eventually resolved) it wouldn't say if it was even connected, which I figured out by hooking up a portable printer to one of the front USB ports.
At about this point, Bob decided to mention that he'd had issues connecting to the Internet as well, even before we got there to upgrade all the equipment. Before we'd started that morning, the IT admin had said he hadn't received notification that any of the computers were having issues connecting. This was odd.
I checked the computer again, this time pulling it out a bit to look at the layout. This revealed the reason behind every issue Bob was having.
The USB cable for the printer was plugged into the Ethernet port, and the Ethernet cable was in an open docking area (for added PCI cards in the rear of the case) with no guard on it. Correction: The guard was broken off by what looked like a hammer and chisel job, and the Ethernet cord was resting in the open area -- basically dangling inside the case. I couldn't believe it.