I've had two episodes, one witnessed.

The witnessed episode occurred as I was commuting one early morning, about 0530. Usually nobody on the street, but for some reason the light about 100 yards ahead turned red. It usually cycles fairly quickly, so I slowed down, expecting it to turn green and I could just start pedaling again. Well, the light didn't change, and kept slowing down expecting it to change, and then suddenly I'm at the stop line and haven't clipped out. Fell to the right. And of course a car pulls up behind me just as I'm falling. Ten miles later, at work and in the shower, I see the big abrasion on my hip. My embarrassment kept me from feeling any pain.

The second episode occurred again on my morning commute. I had stopped to take a picture of the sunrise, and had clipped out my left foot and had it planted firmly on the ground. As I'm snapping a couple of photos, I want to plant my right foot on the ground, so I go to put it on the ground, completely forgetting it was still clipped in. Lost my balance and fell over to the right. No harm.

Luckily, I'm still pretty consciously aware of being clipped in, and unclip early to avoid falls. And despite my falls, there's no way I'd ever go back to platform pedals.