Until a few weeks ago, I've never made a serious attempt to look for a green flash on the setting sun. My horizons are treed and mountainous. And when I have had access to an ocean or flat desert horizon, there have either been clouds or other things to do.
While on a week-long cruise in the Carribbean last month, one evening offered a beautiful sunset where the distant clouds didn't quite conceal the horizon. I took the opportunity to shoot some photos over the railing to see if several miles of thick, refracting air would throw a green sunbeam at me. It was a beautiful sight, but I didn't see a lick of green. At least that's what I thought. After checking the images when we got home, I noticed my final image had an interestingly tinted sliver of sunlight in it. That last little gasp of greenish light might be a visual contrast effect with the orange sky, but it does measure ever-so-slightly greenish in Photoshop. (I didn't apply any unsharp masking to the photo--but random color noise might play into it.)
I know, it's pretty sad...
Here is an animated gif image (470K): Animated Sunset.