I broke a shift cable once, on a rear derailleur. From what I remember, it snapped at the pinch bolt. It was very old, the bike dates to the late '80s and hadn't been serviced much till I got it, so the cable could have been 25 years old. I also broke a Sturmey-Archer 3-speed shift cable once, again, it was very old. I've never broken a shift cable that was new enough to have been fitted by me. You must have just been very unlucky.