When you say "caused the cable to fail", do you mean caused the cable to break?
This is how it looked after breaking:
Wife mentioned during a ride that shifts weren't happening, and I was able to get a few strands hooked to the FD, to get us through the ride. When I unhooked it at home, it came apart.
I'm embarrassed, because I should have inspected the cables periodically at which point I would have noticed the fraying. Guess I've got a new item for my checklist.
Based on your info, my current theory is that the cable was rubbing against a part of the derailleur, which caused it to fray. It looks like it was coming in contact with something. When I put on the new cable, I'll check that out.
Thanks for the help.