Proven Guilty and my personal canon.

[Mira]

I don't think Lea's bargain obligated her to save Harry from his own error blowing up in his face. Mab is a good candidate for having done the fix, but I suspect her reasoning was to preserve an asset she had future use for, rather than to cover any obligation of Lea's.

Have we even established that it was an error on Harry's part?  It's been a while, but I don't seem to remember Harry saying it was an error on his part that broke Little Chicago.  Little Chicago was about details down to the last tree branch for it to work, which at the same time may have been it's greatest flaw.  A city is a dynamic place buildings are torn down,built, and altered on a daily basis.  Streets get torn up or get pot holes, trees fall down and are planted.  If not duplicated in the modal of Little Chicago, then the magic that goes with it has also got to be altered..  So what if no one "broke" Little Chicago?  Then even Toot, who with his buddies covers a lot of area, may have spotted something new in the landscape and with help was able to fix the "error."

[morriswalters]

The problem is presented by in the denouement.  How did anyone know of the existence of LC and who knew it well enough to know it was broken and so how to fix it? Only one character would have known by default.  Lasciel's  Shadow. Mab would have known how to get in but how could she know that it was there and broken?

Bob says something strange.

“It didn’t fix itself,” Bob said. “When I looked at it a few nights ago, the flawed section was in plain sight, even if I didn’t recognize it at the time. When I looked again tonight, it was different. Someone changed it.”

If he recognized it was wrong after the fact why didn't he recognize it before?

Why was Michael moved out of town?  And by that I mean, what was Uriel reacting to?  Molly made use of her free will so Uriel couldn't balance that.  Who interfered in such a way so that Uriel could act? If the error in LC were natural rather than a contrivance of an enemy then why could Uriel act?  And what enemy could have provoked him to balance the scales?

[vincentric]

Do we know if Harry's housekeeping brownies were Summer or Winter Fae?  If they were Winter, then I'd say Mab fixed LC as part of Lea's godmothering. Since Lea watched Harry's place foe enemies, it would have been simple for her to enter a a benign fae while Harry and Bob were out just like the cleaners do. Mab did the same while covering for Lea and spotted and fixed the flaw .

[Mira]

If he recognized it was wrong after the fact why didn't he recognize it before?

 Or as is his way, Harry is beating himself up for something that might not have been his fault. When he looked at it before it was okay, he didn't recognize it because at that point it was okay.  If someone was screwing with him, and Mab could be that someone, or even Uriel, since the whole point of this apparently was ultimately to save Harry from his clash with the Merlin later.  Uriel would have the power to have Little Chicago look normal when Harry looked at it, then cause the flaw that made it fail, then fix it again.

[Snark Knight]

Do we know if Harry's housekeeping brownies were Summer or Winter Fae?  If they were Winter, then I'd say Mab fixed LC as part of Lea's godmothering. Since Lea watched Harry's place foe enemies, it would have been simple for her to enter a a benign fae while Harry and Bob were out just like the cleaners do. Mab did the same while covering for Lea and spotted and fixed the flaw .

The brownies were Summer - a boon from Lily for saving her at the end of Summer Knight.

Lea and Mab are equally free to walk right into his home as benevolent visitors too, though.

[Mira]

The brownies were Summer - a boon from Lily for saving her at the end of Summer Knight.

Lea and Mab are equally free to walk right into his home as benevolent visitors too, though.

  When you think about it, Mab and Lea cannot freely cross Harry's threshold, or at least I do
not remember them doing so.  The Brownies have permission as his cleaning service, but it is unlikely that Harry would have allowed them into his lab. Since he has never had an on page conversation with them we do not know their level of understanding as we do Toot.  So the one guy that can go anywhere he wants I think is Uriel.

[Ed0517]

  When you think about it, Mab and Lea cannot freely cross Harry's threshold, or at least I do
not remember them doing so.  The Brownies have permission as his cleaning service, but it is unlikely that Harry would have allowed them into his lab. Since he has never had an on page conversation with them we do not know their level of understanding as we do Toot.  So the one guy that can go anywhere he wants I think is Uriel.

Ah, but does going via a Way count as a threshold? Did Harry, or more likely it would have to be Michael, invite Mab that Christmas Eve? That's a MUCH stronger threshold, or did she just have to leave most of her power outside. Harry's crossed a threshold for the Ordo Lebes (sp) uninvited to show he had no eveil intentions - he basically disarmed himself.

[Ed0517]

Bob says something strange.

It didn’t fix itself,” Bob said. “When I looked at it a few nights ago, the flawed section was in plain sight, even if I didn’t recognize it at the time. When I looked again tonight, it was different. Someone changed it.”

If he recognized it was wrong after the fact why didn't he recognize it before?

Maybe he noticed the change as DIFFERENT, instead of WRONG, at first. It was only until he did some research he found the newer configuration was correct, and the older one wrong.

As Mira pointed out - cities change. As an example - suppose I made Little New York.  I made it in 1960. It could have been right at the time. 1980 rolls around - the Trade Towers have been built - and Greenwich Street was CUT OFF at the southern edge of the Trade Center complex, to resume again NORTH of the complex. In the prior model, it was a continuous run. Mab fixes it. My Bob notices a CHANGE and asks me for some data. I give him a Hagstrom Atlas (no electronics!) with a city street map - and he sees the new construction changed the map grid. My 1960 model was wrong in 1980.

(and just to screw with the maps again, they changed the map grid during reconstruction of the area, so 1960 .NE. 1980 .NE. 2022  )

[Snark Knight]

When you think about it, Mab and Lea cannot freely cross Harry's threshold, or at least I do not remember them doing so.

They can go anywhere they want so long as their intentions are benevolent to the homeowner. The scene with Cat Sith appearing in Thomas' apartment without his permission in CD explains those rules.

[morriswalters]

All cities change on a daily basis, so no map can or does try to keep up.  Entropy happens.  LC would be useless if it was that important.  Butcher made a narrative decision to have the device be broken. It's when he makes the reveal that is most important IMHO.  The story is over and he's recapping and explaining as part of the denouement. Why tell us then, when it no longer makes any difference?

And can someone explain to me Lasciels's purpose in the plot?  She doesn't do anything useful, at least on the surface. Does Butcher think we forgot about her?

She jump scares Harry in the car just before he is hit.
She throws a hissy fit when he uses LC.
She talks to him in the church.

Use the threshold the way Butcher does.  He uses it to nerf magic. If he wants a character in then the threshold lets them in. If he wants them out then they are out. And if the story demands it there is a workaround.

[Mira]

All cities change on a daily basis, so no map can or does try to keep up.  Entropy happens.  LC would be useless if it was that important.  Butcher made a narrative decision to have the device be broken. It's when he makes the reveal that is most important IMHO.  The story is over and he's recapping and explaining as part of the denouement. Why tell us then, when it no longer makes any difference?

And can someone explain to me Lasciels's purpose in the plot?  She doesn't do anything useful, at least on the surface. Does Butcher think we forgot about her?

She jump scares Harry in the car just before he is hit.
She throws a hissy fit when he uses LC.
She talks to him in the church.

Use the threshold the way Butcher does.  He uses it to nerf magic. If he wants a character in then the threshold lets them in. If he wants them out then they are out. And if the story demands it there is a workaround.

She is there, she is a source of information, more so than Bob, more importantly, and it is easy to miss, she is becoming Lash.  There is one line, one line only but could be the most significant line in the whole book in my opinion, has huge implications for the plot down the road in future books. Harry is in the chapel of St Mary's on page 288

Lasciel's voice came to me, very quietly, and sounded odd.  Sad. It is beautiful here.

That,in my opinion was a turning point or the signal of one, Lasciel is changing.  Lasciel would never have made those observations, she was all about manipulating Harry to see things her way even as she was helping him. This was regret, an awakening of long lost memories of what it was like before her Fall.  Lash was about to be born, not too long after that she begins to help Harry play music, even compose, something else she has never done since her fall, supposedly hadn't missed but forgotten how much pleasure she got from it.

[Snark Knight]

She is there, she is a source of information, more so than Bob, more importantly, and it is easy to miss, she is becoming Lash.

She also noticed hints that Molly was crushing on Harry at the time, which ends up being relevant.

[morriswalters]

She is there, she is a source of information, more so than Bob, more importantly, and it is easy to miss, she is becoming Lash.

I wish you would point me towards any information Lash gave Harry in Proven Guilty.  It eludes me. In point of fact.

There was nothing I could do to prevent Lasciel from seeing and hearing everything I did, or from rummaging randomly in my memories, but I had learned that I could at least veil my active thoughts from her. I did so constantly, in order to prevent her from learning too much, too quickly.

Sounds kinda one way to me. But for me this answers the question, who could have known. I won't quote the second instance but it answers the question, how could LC been damaged.  Bob couldn't see it because he had been told not to.  Lash took Harry's body for a little walk. When Bob tells Harry about it here is what the conversation looks like.

I looked again tonight, it was different. Someone changed it.”

“In my lab? Under my house? Which is behind my wards? That’s impossible.”

“No it isn’t,” Bob said. “Just really, really, really, really, really, really difficult. And unlikely. He would have had to know that you had a lab down here. And he would have had to know how to get around your wards.”

This is interesting.  If it were sabotaged, the person doing so would have to have as much information as the person who fixed it.

[Mira]

I wish you would point me towards any information Lash gave Harry in Proven Guilty.  It eludes me. In point of fact.

Perhaps because it isn't about any information  Lasciel gave or didn't give Harry, it is the first concrete evidence that Lasciel is changing into Lash.  It is easy to miss as I said, but it is there.

[morriswalters]

I missed one event.  Lash helped Harry escape from the Fool Moon Garage.