Factual error: The musicians in the (real) Band of the Coldstream Guards that plays Happy Birthday for Christopher Robin are wearing their 21st-century uniforms and medals.
Factual error: In the beginning of the movie, during Dickens' visit to America, you can hear them playing the melody from the song "I'm a Yankee Doodle Dandy" written by George M. Cohan in 1904. This was 61 years after the events in the movie. In the ending credits they cite this as "Yankee Doodle Dandy" which was written many years before. But, that was not the melody that was being played.
Factual error: The RAF Dakota that flies Robin and Diana back from Kenya in 1959 has American markings and D-Day invasion stripes, marking it as a heritage aircraft rather than a service aircraft of the era.
Factual error: Allegra was said at the end of the film to have died aged 10. She was actually 5 years old when she died.
Factual error: In the movie the professor's assistant says thst the professor was in OSS in the great war (WWI). OSS was first formed in 1941 by President Roosevelt.