One of the best things that ever happened to me on a randonnee was discovering that I had stashed a spare pair of gloves in my seat bag. My other gloves were soaked and it was snowing. Finished that one, but there were times when I wondered if I was going to live through it -- and I mean that in all seriousness.
I'm a lot more willing not to start nowadays. I was going to ride a 600k last year that promised thunderstorms for the entire ride, but I was one of only two that signed up for it and the other guy decided not to ride so I didn't either. I'm no longer willing to start when either high winds or frozen precipitation are likely.