Corrected entry: While fighting the Hydra in the Parthenon, Percy, with Luke's open shield on his arm, runs to hide behind the statue of Athena. He then stumbles and knocks the shield off his arm, sending it flying. However, he is never seen picking it up, nor did any of the kids have it when they raced to the door to leave the Parthenon. Percy just suddenly has it later in the movie.
Corrected entry: In the scene where Percy and his friends are fighting the Hydra, Percy begins to wildly swing his sword to cut of their heads. At one point, he slices off the middle head which breathes fire, and it grows back. This is incorrect as the middle fire breathing head cannot be chopped off.
Corrected entry: When Percy, Grover, and Annabeth go to meet Hades, they find his wife Persephone with him. This is impossible, as Persephone only lives in the Underworld during the winter seasons and the movie is repeatedly proved to take place during the summer. In Greek mythology and The Lightning Thief, the seasons are caused because her mother, Demeter, is sad that she is away from her daughter and causes the crops to wither.