Factual error: If you take the train from Paris to Cannes you won't pass the Eiffel tower.
Continuity mistake: Kevin Kline is walking through a vineyard. His shoes change from boots to loafers in the same scene.
Audio problem: All Air Canada employees speak with a very pronounced French accent rather than the expected French-Canadian accent.
Factual error: Kate and Charlie live in Toronto. When we first see them in their car, at the beginning of the movie, it has no license plate in the front. In Ontario, cars are required by law to have a plate in the front as well as in the back. While some jurisdictions may be lax in applying their two-plates law, Ontario isn't.
Other mistake: During the scene where Luc and Kate first meet on the plane and are talking, Luc gets up to go to the bathroom (the second time). When he returns to his seat, the plane has landed, Kate is sleeping, and the other passengers are deboarding. There's no way he would have been allowed to remain in the rest room during a landing.
Continuity mistake: In the restaurant at Cannes, Meg Ryan stumbles backward over a dessert cart and lands with her face clearly on the floor. After a cutaway to the diners' reacting to the crash, Meg's face is squarely in a chocolate cake. Maybe she just became hungry after the accident.
Continuity mistake: When Kate comes out of the bathroom after taking her shower in Cannes, she has thick eye shadow on. That's some shower she's had...
Suggested correction: Her hair is also dry and styled. It's obvious that she did her hair and makeup before leaving the bathroom.
Continuity mistake: Kate is speaking to Charlie on the phone when he says he's met a woman. The length of the wire changes from short to very long instantly.
Continuity mistake: When Meg Ryan and Kevin Kline are having breakfast in the train, the label on the water bottle changes position between shots.
Continuity mistake: When Kate and Luc are chatting in the plane, a shot from the outside shows the man behind sleeping, with his face turned towards the window, but the next shot, inside, shows his head facing straight forward.