The character Oliver Ashford states he has a commission in the Royal Navy purchased for him. Impossible as commissions have never been sold in the RN, which at the time was the nearest thing to a meritocracy the world had seen. Grease a few palms to get him in as a midshipman? Maybe. As a captain, never. Fleet boards had to be sat, idiots weeded out.
Commissions could be bought and sold in the cavalry and infantry but not, for instance, in the Royal Engineers. Soldiers could be lost to incompetence but not ships or bridges. Far too valuable.
Engagement rings were not used in the late 1700s. They didn't emerge until the 1920s.
In the real painting Lady Elizabeth Murray wore overall pink gown but in the movie she never once wore overall pink colored gown.
John Daviniere was 7 years younger than Dido. In the film, Dido married Davinier when she was 18, therefore John was only 11 years old at the time.
When Elizabeth and Dido play the piano, it sounds like a large modern piano, not the small 18th-century piano onscreen. Its keys look very old, even though pianos had existed for a few decades, and had just become popular. Most pianos in use at the time would have been fairly new.
During Mansfield's conversation, an electric candlestick is visible on the wall.
Dido and Elizabeth's painting was painted in 1778, five years before the Zong Ship case, while the film shows them simultaneously.