According to the DVD commentary, several of the film's actual crew members appear in the crew of the margarine commercial.
Hugh Jackman took etiquette lessons from 19th-century etiquette expert Jane Gibson (who trained actors for such projects as Sense and Sensibility (1995)). He also studied ballroom dancing and trained to ride a horse for the film.
When Meg Ryan knew that Hugh Jackman was going to spend his wedding anniversary alone, she phoned his wife to invite her to dinner. Reportedly, Jackman had to spend the day working.
When Leopold is chasing Stuart down the stairs and out of his house, Stuart seems to jump a few meters in a second. In the "Director's Cut" version, the two guys actually bump into Kate there.
In the beginning of the movie, Leopold looks back at Stuart during the bridge dedication speech. Behind Leopold's head, a spectator in the crowd waves a 37-star U.S. flag which is the correct number of stars from 1868 to 1877 (8 stars on top, 3 rows of 7 stars, 8 stars on bottom).
James Mangold: the film director arguing with Kate about the likability of the leading lady in the film the focus group has just been watching. This was not in the theatrical version.