Originally Posted by
The Human Car
Around mile 16 my rear shifter cable broke inside the shifter housing (of the Shimano Tiagra for the third time since I got the bike.

) I had a spare but it took some work fishing out the little piece that was left inside the works, anyway by the time I got it all back together Gloria pulled up and we all went to the rest stop together and had some good conversation.
I am now officially annoyed with the cable breaking where it does, it does not do that with the front derailleur so something is amiss.
I had the same thing with my Giant when I purchased it new. A couple years later of dealing with it, the kid at Joe's Bike Shop said it was from being worn, but it did it when new! I think it was a QA thing--inspection missed a burr or something. Anyway, it quit doing it once I purchased a new shifter.
Maybe you'll be lucky, can open it up and find the area in the shifter that's chewing up the cable, but if the bike is under warranty, have them order you a new one. You got it thru an LBS, right?