I think I've always tended to pack for worst case scenarios, be it for backpacking or just travel. Must be the influence of my pessimistic father, who always assumed the situation would be going to hell in a handbasket!

I always have warm enough clothing for the worst case weather; I was able to supply an extra jacket to a friend who came on a week-long August trip in the Sierra with us without raingear or tent (it rained every day! at night he'd roll up in his groundcloth, once or twice he came into our tent for the late afternoon thundershower). He would've survived without it, but I am not that tough so I know I need to bring enough shelter and warm stuff for myself. And we always bring a full first-aid kit.