Years after I had "mastered" clipless, I went out for a ride with an old friend who took me into a completely new (to me) group. We came to a light about 20 miles into it, and I just couldn't get out of my pedal to put my foot down. Fought if for a while valiantly, hopped a bit, finally keeled over into the grass about 10 ft from where the fun started. Everyone was laughing at me like I was some noob. It was quite a show. It turns out I hadn't properly greased and tightened a cleat screw on my new-ish shoes, and it had fallen out, so the remaining screw had me firmly attached but the cleat wouldn't rotate to click out.

Morale of the story: with clipless, you are never safe.