Sadly, the only two times I've fallen due to clipless have been when I've been at a full stop with a foot down.
Somehow when I wasn't paying attention, the bike started falling toward my clipped in foot, and I couldn't clip out in time.
The first time this happened, I was next to a bus and caught myself. The second was turtle time.


I also fell a couple times when I temporarily switched back to clips and straps while wearing Vans.
With my old Adidas, I could wiggle my feet out of the straps, even when they were cinched down, but not so with the vans. I found that out the hard way.
On the ride back I kept thinking "be sure to undo the straps before you stop" but at the last second thought "hey, I wonder if I can skid with clips!" skidded to a stop, and immediately fell over with both feet strapped in.