I have had a couple of harmless falls on MTB tracks where I came to an unexpected stop and didn't unclip in time. It's been a long time since the last one though.
I did once have a near-miss with potentially huge consequences. The company I work for arranges an annual road sportive for its employees and makes a big thing of it. A guest speaker is brought in to generate interest and motivate the riders. On the day of the ride I was on my bike heading slowly towards the start - I was cycling slowly because there were pedestrians and other cyclists moving about. A pair of pedestrians in particular caused me to slow to a near stop to allow them to pass; I unclipped my left pedal but over-balanced to the right, causing me to start leaning towards the pedestrians with the likelihood of hitting them. Luckily I run my SPD pedals not too tight so the panicked attempt to unclip on the right worked just in time to avoid any contact, but not in time to avoid my embarrassment. The pedestrians looked a bit bemused and not a little amused at my predicament. Ladies and gentlemen, the pedestrians were Lizzie Armitstead (as she was then) and her agent - I nearly ended either her career or mine.