Other mistake: When Catherine breaks into Crown's house, her assistants immediately get to work on the security panel. When the first assistant touches the panel, it falls off the wall and he has to quickly push it back into place.
Other mistake: When Thomas' keys are taken and copied, the name "Medeco" appears on the keys. The duplicating machine used is for standard keys and cannot be used for Medeco keys.
Other mistake: When Catherine breaks into TC's house and is searching for the Monet there is a scene where she admires The Mirror Of Venus, the footage of this scene has been flipped because the painting is a mirror image of it's actual composition.
Other mistake: Near the start of the movie when Thomas Crown enters his office for the first time, he uses a magnetic swipe card to access the elevator to his office. After he swipes the card, you can hear a 'ding' and the sound of elevator doors opening but you'll notice as he first inserts the card into the reader that the doors are already open. (00:05:30)
Answer: On the commentary track for the DVD the director states that he has no idea how Thomas Crown stole the final painting.