My worst experience as Cubmaster has been the pinewood derby. We had a guy bring in an EBay car but he was disqualified per our rules. Another boy asked his father which car was his. The CC heard that and the car did not race. Many others look too nice for a tiger to make, but you never know. My son took it seriously and sanded until it was somooth. He found out that if he used sandible auto primer, he got to spray paint more, so he was able to get the paint job very smooth. (notice, I said "he was able.")
We print out drivers licenses for the boys and there is a statement on the back that says "I promise that I made this car myself." The parents and boys knew about this in the beginning, so it was in the back of their heads while making the car.
I was struggling with too much parent involvement in the cars, so I came up with a "Geezer Derby." The parents make their own cars and race them at my house in the evening of race day. It is an adult event with no children, and not a scouting event, so everyone brings appetizers and refreshments. They have to follow the same dimensional rules as the boys. We have awards just like the boys. The most coveted (to award, not receive) is the "Car most likely made by a Cub Scout." Not only do the parents let the boys work more on their cars, but the leadership of the Pack has become a more close knit group. Everyone wins.
We are going to hold a "no rules" derby for the Boy Scouts next year. They still want to make cars. The only requirements are that they start with the block of wood, it fits the length and width requirement for the track, and it weighs 5 oz. They can do what ever they want with it after that.