Got a "we don't have Internet" call from a hospital medical testing lab early this morning. An ancient 10 Megabit hub seemed dead (don't know if it actually is, but it definitely couldn't handle the situation). I replaced it with a new 100 Megabit switch. That switch promptly went bananas.
I Started unplugging stuff looking for the source of the problem - it didn't matter, the network was totally non-functional so it wouldn't interfere with anything. Soon got the right cable.
The cable went to a work area used by both the lab and the hospital. There had been a hospital guy doing some stuff there on the weekend - he said he hadn't done anything at all with any cables.
So I guess it was just the Wire Pixies who plugged both ends of a patch cable into the same local 5-port switch.