I had a run in with the ground today on my road bike about 7 miles in and had to miss the rest of the 30 miles. It wasn't my fault and nothing got too damaged on the bike, but the rear derailleur hanger was badly bent and the cable housing on it was destroyed, so the drivetrain didn't really work at all and I had to get San to come pick me up. I was worried I might need a new derailleur but I went by Outback Bikes today and as usual, they took care of me. $5 later I have a new cable and housing, the hanger is straight again, and the derailleur is all in one piece and didn't need replacing. (Hoorah for free service after buying a bike from them!)
Lessons learned today:
Chances are almost 100% that if your bike is not of the current model year, the bike shop that sold you the bike will not have any replacement derailluer hangers in stock, even if it's last years model.
If you are riding with people and their quick release levers are turned in such a way that they might grab on to someone elses levers or cables, suggest to them that they adjust them instead of just not worrying about it!