MK4 4Motion rear suspension arm sheared - why?

[big ironman]

My Mk4 4Motion went into the repair shop (VAG Indie with good reputation, always been great) for MOT, and the OS/rear spring had broken at the end (not noticeable when driving). The car is used as a spare so about 2k miles per year. Both springs were replaced. Driven about 25 miles (some motorway), pulled into drive this evening and a huge bang - rear offside sank, so I got out to see the wheel folded up inside the wheel arch. The lower arm has sheared just by where it is bolted to the axle hub assembly.

Slightly concerned that this should happen just after the car is handed back - I guess it is possible that the arm was damaged before (but surely the spring replacement requires removal of the arm?), and should not the repair shop have checked for and noticed damage? Alternatively, is there anything (like incorrect fitment) that could cause this to crack and break? Also had rear discs and pads replaced.

I'm wondering if this is a common failure (I've not found any similar with google), and also likely causes. Advice on how to approach this with the repair shop appreciated.. if the break had happened 5 minutes earlier then I would not be writing this post, but with that as a backdrop I'm fairly sure they will try to dodge any fault. I'm not looking to sue them or anything litigious, but I am concerned that the repair/recovery for this might be costly .
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[onzarob]

It may of been cracked already, not always easy to see through the grime, I would suggest just contacting the garage and telling them what happened, I'm sure they can fix it. How many miles has the car done?


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[zollaf]

car is at least 12 years old, metal corrodes and gets weak. we in the trade are seeing a lot of rust under cars lately, mk4 golfs rear axles ( front wheel drive) rotting through, ford focus's falling to bits, a kias that are only 5 years old needing rear suspension rebuilding. blame it on driving the car especially when its wet. you cannot hold the garage responsible for it though, but the rear has been apart so moving it may have stressed an already weak but un noticed weak spot. just count yourself lucky it happened not on the motorway or you could have typed this from beyond the grave. best to get it recovered to the garage and get some new bits fitted, they will be as surprised as you but also concerned its happened after they have worked on it. i also presume the car has been sitting for a while and only just driven for the mot. maybe since its not used you should get them to go over the whole thing with a view to checking everything underneath. suspension arms can get weak but be hard to notice or actually fail an mot..

[big ironman]

Thanks. Car is 15 years old, but has been well maintained, and re your suggestion, I always ask the garage to check it all over and do whatever is required / fit proper parts if anything looks suspect. On the failure point, the 'tear' (it is a pressed steel part at the end) was bright with no significant corrosion - it looks like the joint, which should pivot, did not, so the metal broke. It's used as a town run-around these days (a lot easier to park than a Q5), so does not sit idle for weeks on end - usually used 3 times a week at least. Let's see where we get to. And yes, I am grateful that luck was on my side.