It makes no sense that Allie could be King arthur first she is a girl King arthur is a guy second she doesn't have a love triangle between anyone and she probably would have held a sword before because her parents are professors of midevil literature and they probably would have let her hold one before and would have noticed if it had turned into Excalibur.
The plot of this movie differs from other King Arthur stories, as they claim that Mordred was Arthur's brother, where as most other literature state that he is Arthur's illegitimate son.
When Allie runs into the forest to tell Will that he is King Arthur, the moon above her is in a crescent. However, later on during the football game, the moon is now a full moon.
Brittany is constantly jogging throughout the film, and doing long distance runs to train for track. However, she runs the 400m, a sprint, and therefore should be encompassing far more sprinting and speed work than distance training.