Regrettably I've also seen agile/xp go wrong and quality drop. It was more of a management problem, the number of tasks completed were pretty much the only metric devs were evaluated on. So management got what they rewarded, fast task completions. Quality dropped. Management didn't consider in dev evaluations tasks reappearing in future scrums because they were not done quite right the first time. It was quite Dilberte-sque.
Which is what pretty much happens where I work. So for example we had this communication protocol that probably should have taken a month to design implement and a week to test. But oh no, we need it in 2 weeks even though the hardware doesn't actually work. That got pushed off to one dev who basically had a week to work on it. But of course the manager didn't notice the guy was sick and couldn't work on it.(He said he'd work from home but that was just a fantasy.) So guess what happened when it was decided hey you can finish it up since he's out today. Pretty much I implemented and unit tested what I could in a day.(Based off of a toy version of the protocol which basically only handled the perfect case.) Suffice it to say it doesn't really work and nobody seems actually interested in fixing it. (Even though I've given them some advice on some extra code that would actually solve a lot of issues with it. Should I mention the customers notice it doesn't actually work?) God I hate "agile"
Umm, sure since you interrupted me in the zone I might as well help you, what is it?
I can't install this USB device?
(thinking to self)You're supposedly a tech savvy IT professional with a decade of experience and the first thing you think of when you can't get a 3rd party USB device working is talk to software engineering since hey you know, co-location human interaction. Oh and you plug it in and it shows up as a serial port
Step 1 Display something and let the user/ai enter a response
Step 2 Always reject every response
See, works as well as those 2 schemes and is much easier to implement.(I'm only being somewhat sarcastic btw.)