Continuity mistake: When The Rock first enters the scientist's room, the scientist explains to the child about his magic powder. This is on a device that's slowly turning and sifting off a little of the powder at a time, yet during one cut away and back, a lot more powder seems to have disappeared off the mechanism than it would have pushed off itself.
Continuity mistake: In the scene where Cassandra and Mathayus get out of the well after falling through the tub. When they are showing Mathayus from the back of his head his hair looks wet all over but when they show his face, the hair on his head looks dry while the long parts of the hair is still wet. His face has also been dried off when last seen.
Continuity mistake: When Balthazar knocks Matathyus down, Matathyus falls face down and his hair gets covered with white bits of dust (sand or whatever). A brief second later, when he stands up, his hair is clean and shinny, but he hasn't had time to clean it, or even made a single move to take the dust away.
Continuity mistake: After Matathyus (The Rock) finishes fighting with Balthazar (Michael Clarke Duncan), Balthazar is sort of sitting down while Matathyus is standing. You see blood all over Balthazar's mouth. Then the shot goes back and forth between Matathyus and Balthazar three times. Each time there's not the same amount of blood in Balthazar's mouth.
Continuity mistake: When Mathayus is launched out of the catapult, he is shot out of the window and lands in another room in the castle. A few shots later, the catapult is shown again but it is in the cocked position. The scientist was busy with his gunpowder and avoiding the guards - he wouldn't have re-set it, surely?