Factual error: In the second-to-last scene, Rachel is leaving Singapore for New York. Nick goes onboard to propose and convinces her to stay in Singapore for one more night. However, this is not possible in Singapore due to strict laws which state that those who enter the departure area must have a boarding pass and intend to leave the country. Those who have a boarding pass but no intention to depart will be prosecuted. In 2018, 40 people were arrested for various offenses, such as sending a friend off or meeting their idols.
Suggested correction: Nick's family is extremely rich, powerful and famous in Singapore. That might have been enough to get him special treatment or an exception to the rule.
Factual error: The plane used in the film to take Juliet back to London towards the end of the film is a replica of a Dakota C-47. She is painted with black and white "invasion stripes" on both the upper and lower surfaces of the wings - approved in May these were only widely put into use for D-Day in June 1944. After one month the upper surface stripes were ordered removed, and by the end of 1944 they were ordered completely removed. By 1946 when the film is set they would have been long gone from any serving aircraft still flying.
Factual error: Simon receives an email notification at the dinner table from Blue on his cell phone. The cell phone screen indicates that the time is 3:20, which is inaccurate for a mealtime because it is already dark outside and crickets are chirping when he excuses himself to go outside to get some air. (00:28:10)
Factual error: When Buster joins the card game in the saloon, he looks at the hand of the player that had just left. Given the circumstances in the saloon, the cards cannot be spot clean and white as fresh snow. Also, they seem to be plastic cards, which did not exist yet back then.