The Online Photographer has a post about The Photo That Got Away. About lost photo opportunities.
One immediately flies to mind for me: I was on the street in front of my favorite lunch pub, called The Brass Cat. (Very, very old pub, used to be called The Golden Lion, but the customers always called it The Brass Cat, so they adopted it.)
And two workers were working on something, one on a ladder. And his butt exactly covered the Br, so the sign said "The (ass)ass Cat".
I actually did have a digicam in my bag, but I got it out too late. I really should have asked him to go up again, I wouldn't have had to tell anybody...