Okay, this is a crackup only because I had to diagnose a co-worker's connectivity problems a couple of weeks ago. I'd just gotten her a Dell laptop / docking station (she'd previously had a desktop).
She decided she didn't like where I had put the dock on her desk, so rearranged everything, to include rerouting cables. When she was done, she had no connectivity and called me. She had the cat5 end plugged directly into the laptop ethernet port (disabled while plugged into the dock) and a USB cable jammed into the ethernet port on the dock.