Maybe this happens more than I think it does, but I have never heard about it before.
I was riding home from work yesterday and my shifting was all over the place. I'd shift up and it would shift down, I'd shift down and it would shift down 3. So when I got home I went to take my panniers off and index my gears and saw this:

That stretched my cables out in a hurry. So now I need new cables. This happen to anyone else?