I often find myself in a situation where having a camera ready to shoot is not just a good idea, but necessary to my peace-of-mind. This is evidenced by this pic that I grabbed on the spur of the moment.
I was busy picking up some of my dog's poop when I heard the Harley's throaty grumble. I dropped the poop bag, ripped off the lens cap and caught the biker as he made the street corner. It's not great but it'll do for a quick click!