Continuity mistake: In the hospital Stuart tries to grab his neighbor's phone. When he detaches him from his life support and alerts the nurse he holds the receiver in his right hand. The camera changes to a shot from behind, and he picks up the receiver again with his right hand.
Continuity mistake: Toward the end of the movie when Leopold is making his speech of who he is going to marry, there is a shot of the crowd in the room. In the very back there is a person that runs in and stumbles about. Then there is a close-up shot of Kate coming in and stumbling around the exact same way.
Continuity mistake: When Kate is on the top of the roof with Leopold eating dinner, you can see her dress has straps on her shoulders. When the camera shoots back and forth from Kate to Leopold, you can see that the strap has fallen off her shoulder and shows her bra strap. However, when they go back to Kate it is back on her shoulder.
Continuity mistake: Stuart has pictures from the ball at Leopold's that show Kate standing at the back of the room looking in the room. At the end of the movie, you can see the set up for that picture. However, for Stuart to have *taken* that picture, he would have to have been standing on the platform with Leopold to take the picture, but he's not there.
Continuity mistake: When Kate comes back home after the punctured dinner with J.J., her hair is freshly done. Even if she had ample chance to do this, it's out of the question that the rather messy style she had chosen for the dinner was not intentional on such an important occasion, and there was no reason to change this afterwards.
Continuity mistake: Leopold tells Kate that by his 30th birthday he had "become a blemish on the family name", meaning he must be at least 30 in 1876, making his birth year at least 1846. When he goes forward in time to 2001, he is said to be at least 148 years old, making his birth year 1853 (2001-148) which would make him 23 years old.