Velma was hiding in the purple airplane, but after the Red Baron made the plane fly, the plane was green.
When Fred, Daphne, Velma, and the Three Stooges were chasing the Red Baron the Stooges were sitting in the front seat. But later, Fred, Velma, Daphne are the ones in the front seat.
When Curly Joe gets into the dusted plane to chase after the Red Baron, he is in his overalls. But when Scooby goes up to tell him how to fly, he is wearing his flight jacket and goggles.
The mill depicts having a door right next to the farm. This is impossible, since they are geographically in the opposite direction.