The film is riddled with numerous inaccuracies about the biographies of King Mongkut and King Chulalongkorn (see trivia), causing the film to be banned and shunned in Thailand/Siam as libelous and slanderous.
Tuptim's play "Small House of Uncle Thomas" is an inaccurate hodgepodge of characters and scenes from the original book, but this can easily be understood as her best interpretation of the story as a new speaker of English who wants to use the story for her own purposes to change the King's heart. However, few of the references to Buddha or Buddhism within the song are depicted accurately and shows a clear Western interpretation of the religion. Some specific examples are that Buddhists do not view Buddha as God but rather as the founder of their teachings and the first to attain Enlightenment, they do not therefore pray to Buddha for help and guidance, they do not believe in angels, and they do not believe that Buddha calls them to his presence when they die; all of these are strictly Christian beliefs, most likely used in the story for the purpose of relating to a Western audience.
In "March of the Siamese Children," when the crown prince appears the King greets him with his arms crossed. After the prince is presented to Mrs. Anna and starts backing away, a shot of the King shows him very proudly looking at his son with his hands behind his back. In the next shot, however, the King's arms are crossed again.
When the children are being presented to Anna, one of them turns and walks away instead of backing away as would be expected and the King reacts with surprise. The child recognizes the mistake but then continues to back away; however, in the next shot as the next child approaches Anna the first child is seen to be in front of Anna again, this time backing away properly.
The map of the world shows a separate Norway. It was not until 1905 that Norway separated from Sweden and became an independent kingdom. Nonetheless a distinct country called Norway did exist, even if it wasn't independent, and it is highlighted in a different color on the map because it leads Anna to teach the children about Norway.