The rules for Formula One racers require them to have a steel cable connecting the engine to the airframe. This cable keeps the engine from completely falling off. While watching a Formula One race at the 1974 Reno Air Races, one racer had a prop blade break off. The engine ended up hanging between the gears. Lucky for the pilot this happened while turning onto the straight section of the course aligned with the runway. He landed it on the engines.
After there were a few more episodes of this type of accident, metal prop were outlawed for these racers. The owners were making the blades so thin that they became susceptible to flutter. Now only wood props are allowed.