Continuity mistake: When George (dressed as Blackadder) is in the cellar and about to go upstairs, he has one cup on his tray, however when he arrives upstairs there are two cups on the tray.
Deliberate mistake: There are several historical dates wrong in this episode. For a start, Prince George didn't become Prince Regent until 1811, and yet this is set before the Battle of Trafalgar in 1805 (as Black Adder gives Wellington some advice on it). Similarly, Wellington didn't get the title "Duke of Wellington" until 1814. Again inconsistent with being pre-Trafalgar.
Deliberate mistake: There are a lot of jokes with the name of the book "The Prince and the Pauper." However the book was published in 1881, 51 years after the Prince Regent died.
Factual error: When Blackadder has disguised himself as the Prince and is speaking to Wellington, Wellington says, "I have with me here the most recent briefs from my General in the field". Blackadder, deliberately misunderstanding, replies "If you would like to pop them in the laundry basket". Blackadder is referring to the short, tight style of men's modern underwear, known as briefs, which were not invented until the 1930s.
Answer: Tony Aitken played the Balladeer. Blackadder wants to kill him because he annoys him and usually sings songs about what a fool he is.
Captain Defenestrator