Difference in the angle of the ship's gangway as Steele approaches to go aboard.
Albrecht Dürer's "Adoration of the Magi" (called "Adoration of the Kings" in the film), and the forgery that is passing for it, are shown as paintings on canvas, which people roll up in several scenes. However, the real painting is on a wood panel.
A person self admittedly obsessed with art would not have a picture on the wall that is double in size to the original. Vermeer's 'Woman with pearl necklace' is only 55 cm high and in the movie it is over one meter high.
Barton addresses a character by her real name, not her character's name. It happens about 10 minutes into the movie, when George Steele's lecture is just ending. Among the attendees, Barton turns around, sees Mary, and addresses her as "Miss Ware." The character Mary's last name is unknown, but she's played by Mary Ware.
The ship Pat O'Brien goes to is riding so high at the dock that it is obviously empty, even though it is due to sail at 1:00 a.m. with a cargo.