Continuity mistake: Right after the debate in the House of Commons, Robert talks to Gertrude in the hall way. Behind them is a statue head. Several shots later that statue head magically moves to the opposite wall where Arthur "talks" to it.
Continuity mistake: Before the party scene, there's a little musical montage where all the occurs can be seen getting dressed. There's quick second long shot of Lord Goring, played by Rupert Everett, placing a bright red flower in his buttonhole. A few seconds later, he's placing a white flower in his button hole. Its really quick but it unmistakably Lord Goring.
Continuity mistake: When Lord Goring and his father are riding through the park in a carriage, Sir Robert is talking but his lips don't move and/or move out of sync. with the dialogue.
Factual error: In the scene where Arthur tries to convince Gertrude to not force her husband to "retire from public life" you can see (in the close up shots) that his right ear has been pierced twice which is inconsistent with the time period.
Factual error: The movie, set in the late 19th century, shows in its first shot a naked English lord in bed. On his arm, you can see the scar of a vaccination shot, even though this kind of vaccinations weren't available at the time.
Factual error: Throughout the movie the actors are drinking champagne from gold rimmed fluted champagne glasses. That style of glass was not invented until the mid to late 1960s. They would have been drinking from a Champagne/sherbet glass in the 1890's.
Continuity mistake: From a long shot Lord Goring and Gertrude are walking. A man on a bike enters from the right and leans his bike on a wrought iron fence right behind some women. Cut to a mid range shot and the bike has disappeared. (00:34:00)