I experienced this once while flat-spinning toward the ground. I noticed that I could actually see my prop and then realized the engine had actually quit. Fortunately, I was able to land it rather than continue spinning to the ground. Once it stopped on the runway, the prop was slowly spinning. At first, I had NO idea what was up. Thought something inside broke, but the engine merely popped the nut loose and the prop was spinning on the shaft. Whew... and yes, I'm glad it didn't keep running which would have caused damage.

-s