Happened just once. Pulled up to a stop light (red) towards the end of a 60+ mile solo ride... unclipped the wrong foot and "T-I-MMMMM-B-E-RRRR!!!!!!" It was like in slo-mo, too. Managed to get my left hand out to cushion the blow as I went over to the left (into traffic... but everybody was stopped so no biggie). Somehow twisted my left ankle badly as a result and ended up with a grade-2 sprain. Rode home and walking was no fun for a couple weeks afterwards. Riding was OK though... just a chore remembering to unclip out of the OTHER pedal for that time.

Nasty scratch on my Ultegra 6620 pedal but no other damage... other than my ankle and ego.