While mountain biking I had mud freeze in some Shimano clipless pedals. It was below freezing, I was on a flat, mostly smooth fire road, and I was coming to a planned stop. My left foot came out fine, but my right was stuck. Naturally, I fell to the right. At the time, I was rehabilitating a dislocated knee, so I wasn't too happy about falling on that knee.

Since that day, I switched to Egg Beaters and haven't had a single issue.