When Allie and Will first meet at the water fountain, Will speaks, and you can clearly hear a British accent. Next time he is hear the next time we see him, his accent has gone American.
When Allie first goes running in the movie her sweat pants change length.
When Allie's mother drops her off on her first day at her new school the clock on the tower show 3 o'clock. This is really late in the day to start school. Also her mother says that she'll pick Allie up at 3 o'clock, so Allie should get straight back in the car.
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.
In American high schools, track is a spring sport and football is a fall sport. It should be Cross Country being run during football season.
When Will comes over for hamburgers, Allie's mother starts quoting poetry. She explains it's a poem about how Arthur became the leader Britain needed. The lines come from an actual poem, "The Lady of Shalott", by Alfred, Lord Tennyson. The person being described in the poem is Sir Launcelot. King Arthur is not mentioned at all.
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.
The football field end zone is only 5 yards deep. American football field end zones are 10 yards deep. In addition, the curved goal post support is placed on the rear end zone line. It should be a few yards behind the back line (for safety reasons).
A nearly full moon is seen days apart above Allie's house, unchanged.
The "King Author" character forgets his American accent several times in the movie and his step brother switches accents at the end too.