The idiot lights pooped out so there was no warning?
Probably not much worse than breaking a derailleur or cable mid-race.
Almost 40 years ago we were doing a fast century in Michigan utilizing a then brand-new D/A front der. on our tandem.
It twisted off leaving us on middle chainring and 5 speed cog in the back within the first dozen miles. So much for what was then ultra-light components on a tandem!
By the time we hit the 75 mile mark the rear der. cable frayed and broke about 5 miles from the finish.
Yes, we persevered but not happy with failures.
Stuff happens . . .
Reminds me of a century ride way back on a Motobecane mixte back tandem -- we had upgraded to a alloy FD borrowed from a single. It snaggled up with the chain about 30 miles into the deep boonies. The FD alloy cage must have cracked and it was suddenly just gone. For the rest of the ride I had the stoker call a 'pause' and she would reach down and lift the chain off the wrong ring and set it on the needed ring and very lightly pedal on to take it all the way around the ring so we could again push. Never tried to get it onto the granny ring though as was worried about not being able to reach it. All in all it wasn't really all that bad from the Captain's view of things but did have to pay for a visit to her nail lady for a full clean and redo in dark red polish.