Last saturday, I had this frightening experience. It was Saturday club ride and it had rained the night before and since we ride in the country, there was no traffic to take the wetness away from roads.
So about 10 miles into the ride, we hit this steep (by my standards, perhaps 6-8% grade) hill. About halfway into the hill, I am out of breathe and get out of saddle so I can go a bit faster and finish the climb, BAD IDEA. My rear wheel starts slipping/spinning, freaks me out. That did not fortunately make me fall, because my left cleat got released and I was able to land safely on ground.
This was with a mountain bike with slicks on it, I wonder what would have happened if I was riding my roadie.