Joined to say how much I enjoyed this story. It satisfies a particular part of my scifi appetite that I often don't get from contemporary SF. A hard-science problem and its social consequences? Perfect!
One thing the story did which had nothing to do with the story itself was it reminded me how ANGRY a book made me. This book (actually two books) is the Commonwealth saga by Peter F. Hamilton. I'm going to avoid naming specific characters in the spoiler space below, but it will still be spoily to see the event coming at all, even if you don't know who it's about: Er, never mind, this forum doesn't have the SPOILER tag. Shocking! Don't read between the lines then...
--------------------- spoilers for Pandora's star and Judas Unchained -------------------
The bad guys in these books have the ability to make people sleeper agents, with the compromise buried so deep that the victim can't tell it's happened, even if they are activated. In the story, a woman is so compromised, and given a compulsion to fall in love with a main character, which puts her in a position later to do a VERY BAD THING, the cause of which we don't know for something north of 1000 pages. When all this finally comes out, the woman is sent for "deprogramming". The main character tosses this off as no big deal, he'll get his wife back and he could keep the good stuff - she'll still love him.
HOLD THE HELL ON. The entire reason she fell in love with you was outside of her control. If you're removing the bad programming, you gotta remove the compulsion too, and start the relationship from scratch, AT MOST (better off to let the woman live her life never knowing you; you have WAY too much inside information about her). This destroyed the story for me, because it solidified the idea I was getting during the books that the "good guys" weren't really all that good eitther, and their vaunted society was creepy as all hell.
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Anyway, what I really liked about "for want of a nail" is that it faced the same problem - the unrecoverable compromise of the personality of a loved one (Cordelia), and made a hard choice. That right there is excellent drama.
I didnt really get the problem with Cordelia, now that they knew the uncle was not in his right mind why did have to mess with her programing or revert to the older copy? That extra code really didnt matter anymore.
They found the code that made Cordelia protect Georgio, but not the part that caused her to erase evidence that it was happening. The concern, and this is a classic one going back to HAL in 2001, is that as Georgio gets worse and worse, the compulsion to protect him will make Cordelia go to greater and greater lengths. Say, turn off the air in Rava's compartment if she figured it out and nobody was around to see it. Cordelia would become a danger to everyone.
Perhaps a better AI designer would have figured out how to seperate Cordelia's recordkeeping functions from her core personality, but I suspect that if we ever get an AI, it will be just as difficult to do this as it would be to erase a single memory from a person without affecting anything else. Forgetting things HURTS - it's maddening when you can't think of a word, but you can think of everything surrounding that word. You'd have to do massive "pruning" to avoid that kind of effect, and I think that's exactly what the rollback is for. Capturing a state is a lot easier than altering the interconnections.