I try to always have my big bag with me when I'm out and around. It weighs 25 pounds - and that can be a pain - but you never know when an opportunity will present itself.
I was driving over a bridge near my home earlier this evening and saw some amazing light. I quickly parked at a supermarket, grabbed a body already attached to my 17-40 and walked back onto the bridge.
The light was going in and out of the clouds but I managed some decent images. Here's a panorama of three separate images combined in Adobe Photoshop Elements 8.
Click on the photo for a larger view.