I only had one incident. It was at Tooting Broadway, where I stopped as the lights changed, freed my left foot, and, inexplicably, leaned slightly to the right. I got up, got back on the bike, and waited, red-faced, the hour (or so it seemed) for the lights to change.
I suppose one other thing kinda counts. I was using single sided pedals (A520) in the rain on the commute. I set off from the lights, pushed hard, but my foot was on the underside of the pedal, so it slipped, and I caught my knackers on the saddle. But I didn't fall off.
I've been riding clipless for about four or five years.
I think my lack of problems may be to do with the fact that I previously rode with clips & straps for twenty years, pretty successfully. Compared to them, clipless are a breeze. Once, as I was riding out of Lancaster towards the Kellets, I tightened my straps because I was out of town, and didn't need to put my feet down for a while. I sped round a corner, and found myself behind a queue of cars at a temporary traffic light. I managed to stop, but I couldn't get my feet out, so I fell onto the verge, and lay on my back, bike in the air. I giggled for a long time before I could loosen the straps.