Continuity mistake: During the scene when Thomas Crown is sailing, you can see the weather change from very dark and stormy, to just cloudy, to blue sky and sun, depending on the shot.
Revealing mistake: During the glider scenes, they use aerial shots of stuntmen in a real glider and then a prop/dummy one with a blue screen for the close up shots of Thomas and Catherine inside the cockpit. The actual glider has a second rear cockpit control box that you can see in all the aerial shots.The rear pilot is at least 5 feet back from the front one. In the close-ups, with the prop/dummy one, the rear cockpit box is gone and they show Thomas actually leaning on Catherine's back. The actual glider also has a much larger wingspan, fuselage, and cockpit. Again, you see this during the take off shots and all the aerials. When they land in the field and crawl out of the dummy/prop, the tail is only about 5 feet back from the cockpit.
Continuity mistake: Thomas Crown comes into the police station to identify the thieves. As he is getting ready to leave, Catherine eavesdrops on his conversation as she fixes herself a cup of coffee. She then watches him out the window, drinking her coffee, as he runs to his car. The paper coffee cup changes from a purple swirl design with a little bit of green to a blue and white, geometric border with what appears to be an Asian design.
Continuity mistake: When Thomas and Catherine are having breakfast and his servant Paul brings Catherine's special "green" drink, it changes colors. When Paul has it on the tray and sets it down on the table, it is a very deep olive green. When she goes to actually drink it, it's now a light pastel turquoise blue color.
Continuity mistake: When TC opens the briefcase to steal the Monet, the thumb of his latex glove is ripped wide open, which would leave fingerprints. After he places the painting in the briefcase, the next shot shows him closing the brief case and the glove has magically repaired itself.
Visible crew/equipment: When Thomas Crown exits the museum after having stolen the Monet, we see a couple of strange characters, probably crew members, standing on the ledge of the building just behind the cab he gets into. The man with the mask is the most interesting.