Originally Posted by
Andrew R Stewart
Are you sure this is metal that is fraying? Looks to me as a cable coating that is abrading off. Some "low friction coated" cables are known for this. Andy
+1 and coated cables can tend to slip under the clamp nut which might explain why the derailleur was out of adjustment. Some of us scrape off the coating where the cables get clamped for this reason. This is just a guess though but something worth checking and you should take it back to the shop and let them try to fix the issue.