Factual error: The crescent as emblem on Saracen pennants is anachronistic as it appeared on Islamic military flags no sooner than the 15th century A.D.
Factual error: In the final scene with the New York skyline in the background, the new glass/stainless steel Manhattan complex at 731 Lexington Avenue is seen; this building is just being finished now in early '06. The building stands where Alexander's dept store once was. (On a related note, it is a nice touch on Spielberg's part to overlay the Twin Towers onto the skyline in that same scene.)
Factual error: There is a three prong electrical outlet visible in the Wershba's bathroom. These were not used at the time the film takes place, they were introduced decades later.
Factual error: There are a couple of references throughout the movie to Ralph Fiennes' character as an "American diplomat" "of the League of Nations". The United States was not a member of the League, so there were no "American diplomats", as such, working for the League.
Factual error: In an early scene, a character suggests that a man might be awarded an OBE honor if he wins the 1913 US Open. The OBE wasn't established until 1917.