I had one on holiday. Setting sun over the sea. Seated bloke in the foreground and looking at the view. Then a flight of six seabirds flew over diagonally in a line. I only saw them when I reviewed the photos and an exposure at the right moment would have been a classic. As it is I have shots when they were too far away and as they disappeared over my head.
So in my case the decisive moment is usually the one I missed, but I keep on trying!