I don't know much about planes, but I'm reasonably certain when the propeller falls off a single-engine aircraft that's a major problem. 

What makes this particular incident even worse is that the pilot is at an airshow maneuvering the plane in a planned low-level free fall.  That's when the propeller just falls right off.

This could have and probably should have spelled catastrophe, but this pilot used nerves of steel to direct the plane to a perfect landing.