It has happened once to me with another member of this forum

We found a nice rock and a bunch of leaves and it was a warm night so no harm no foul. We got lucky because 1. it was warm and 2. besides that rock, the only other flat spot in the whole canyon had our tents on them!
I remember the moon finally rose and I got a good look at our surroundings and that just completely dumbfounded me. I thought we had somehow stumbled into another canyon, although that would have been near impossible. I just went back to sleep and when morning came, finding our camp was easy.
Lets hear some more stories of an unplanned bivy!