I had a coil drop out of a glow plug into an engine a couple weekends ago. Finished a nice vertical climb and when I reducec throttle the engine started running very roughly. It died when I cut it back to land, and when I turned it over you could feel binding at TDC. Pulled the glow plug and the coil was gone. Pulled the head and found some small balls of metal on top of the piston, which I removed. Engine ran Ok afterwards, but later that week it lost compression and was retired. I didn't see any scoring on the cylinder liner, but the piston might be or the ring could be broken. Watch that engine for a few days.

Andy