Well, last week I had my rear shifter act up. It wouldn't reach the highest or lowest gears and felt slack. Over the years I've had shift and brake cables fray, but this was a new bike with a 9-month old rear shifter cable. I tried adjusting it, but to no avail. I usually replace these myself, but this is my first "brifter" equipped bike, so I took it to the shop. The tech confirmed the cable was frayed and then asked, "have you fallen recently?" As a matter of fact, my bike rolled out from under me while while I stood waiting for a light to change a day before trouble set in. The bike has shifter cables running across the top bar. He explained to me that he has had experience where a rider falls and a leg or shoe or something snags a cable and stretches it out resulting in slack, fraying or an all out break. I'm not sure if that was the cause, but it seems plausable to me.