At the beginning when Johnny and Flash race down the hill, at first the road is dry then in the next shot is soaking wet.
When we see Johnny's car at the track before the first heat it is blue with a white box for the family garage logo. When he leaves the starting gate the car is blue with a solid white line down the middle. The other two cars are white and green. In the shot at the finish line there are two solid blue cars and one green one but with different markings from the beginning of the race.
In the first race heat, Johnny and Flash have changed lanes by the finish. Also Johnny's dad is referred to initially as 'Mr. Reynolds' when he enters the judging tent, subsequently by his correct surname.
It's commonly believed that the Stephens home is located in Westport, CT yet the station wagon Sam is using has New York license plates.
When Abner, Flash, and a furious Gladys leave the judges tent, no one takes the binoculars and Flash's helmet from the table.