Continuity mistake: When Fred walks up to Excalibur, the design is different than when seen later after Shaggy and Scooby are scared by the dragon. The blade has designs on it, including a gold and green diamond shape, and the hand guard is curved. And the handle is wavy, with the end of it being a gray cone shape. Then after Scooby and Shaggy come running, it is shown again. Only now the shape of the blade is different, the gold and green diamond part have vanished, the hand guard is now just straight, the handle is not wavy, and the gray cone at the end has turned into a sphere.
Picture added by Quantom X