Disney discovered that the shooting schedule at Mount Rushmore would displace the local Hill City High School prom that is held annually at the location. To make up for this, Disney paid for the prom by providing transportation to a new location in Rapid City, door prizes at the prom, and scholarships for the valedictorians.
When they are at Buckingham Palace, Ben Gates tells Abigail that Queen Elizabeth II is not in there because there's no flag at the pole. This hasn't happened since 1997, after Princess Diana's death. There's always a flag attached to the pole. If Her Majesty is in residence there's going to be the Royal Standard. If she is not there, there's going to be the Union Jack flag.
The document briefly seen in Riley Poole's (Justin Bartha) book is an autopsy report for Marilyn Monroe. Certain conspiracy theories prevail that Monroe was murdered because of an illicit affair with President John F. Kennedy.
As stated in the DVD commentary, experts were brought in to match the handwriting of Abraham Lincoln, Queen Victoria, and Calvin Coolidge.
The night before the scenes at the University of Maryland were to be filmed, a half inch of snow fell. The crew then had to use fire hoses to hose down the entire McKeldin Mall and nearby locations to melt the snow for continuity in the movie.