It's been a little while, but I can think of a couple of instances where this happened to me.

1. January 2000. I was at work - at a camera store in fact - and it was approaching the end of the day. The whole front of the store was windows that faced west, so I could see the sunset approaching. I got out early and drove to a nearby lake I knew had a good view. I spent probably 3 rolls of film on that sunset, and only 1 of those images really survived the last 18 years.
   

2. May 2005. My brother (also a photographer) was in town from Colorado, and we were visiting his mother in-law at her property out on a nice big stretch of land atop a hill. While we were inside enjoying a nice craft beer, I happened to look outside and see this rainbow just raging. He and I both scrambled out to our cars and grabbed out camera bags and just snapped away.