I was building 30 cut-to-length patch cables, and ended up miswiring one.
One end was the normal (OR/WT, OR, GW, Bl, BlW, G, BrW, Br), but the other end had a wire out of place (GW, OW, O, Bl, BlW, G, BrW, Br). Normally I would test every cable, but the one in question was for a demo, not production.
Now here is the weird part:
I was playing around with a smaller WatchGuard XTM, and had a live network running. I wanted to play with internet failover, so I ran additional cables from the modems to this new XTM. One of the cables was this miswired cable. Upon plugging the cable in, I noticed that the production network failed over to the secondary modem...weird...but it would come back up as soon as the new XTM would settle down. It did this every time I rebooted the new XTM or I plugged/unplugged that cable.
I noted the XTM didn't agree with the interface and wouldn't connect to the internet from that interface, so I static IP'd a laptop and plugged it in, same result on the production network, but finally found a clue, the status only shows 10MB, so I scrutinized the ends, and saw my mistake.
Here's the question, why would such a tiny fault cause the production network connection to drop? The modem in question is an Arris DG860A