Factual error: When speaking to a member of the the Royal Family in Siam (modern day Thailand), one have to speak "rachasap" or Royal Thai. The movie depicts characters speaking colloquial Thai to the King.
Factual error: The play of Uncle Tom's cabin put on by Tuptim and the Siamese is wildly inaccurate, and bears almost no resemblance to the book Uncle Tom's Cabin other than the names of the characters.