A while ago, I took my chain off to clean it and when I put it back on, I routed it past the wrong side of that little metal tab on the rear derailler cage. I rode like that for a total of about fifty miles, the whole time wondering what was wrong with the bike. Finally someone else noticed. I was totally embarrassed. It didn't help at all that the person who noticed was a good-looking woman either. I'm not likely to do it again.