I'm still very new to clipless....and it happened yesterday for me, on the Tour de Cure Charity ride.
3 or 4 riders in front of me stopped at an intersection, and I unclipped the right side, and went over on left (unclipped side) when stopping.
Talk about a slow motion, awkward, frozen in time moment.....

One of them helped me up, made sure I was OK (minor leg scratch, major embarrassment), and they left me to go climb the next hill.
Two of the guys there said it had also happened to them before, so I guess it does happen !!
I guess my lesson is I need to unclip sooner, and be aware of what foot I'm going to put down....