Factual error: Dr. Johnson completed his dictionary in 1755, 7 years before the Prince Regent was born (1762). So it's unlikely for him to be asking the Prince for patronage.
Factual error: During the theatre play you can see they are using electric stage lights. Nice move for a series plotted almost 300 years ago. (00:01:55)
Visible crew/equipment: In the shot after Blackadder has read up Baldrick's votes a microphone is visible in the upper right corner. (00:18:15)
Visible crew/equipment: During the conversation between Blackadder and Baldrick, as Baldrick explains what he spent the £400,000 on, we can see the shadow from the boom mike moving on Blackadder's fur coat. (00:28:00)
Revealing mistake: When Baldrick is on the metal spit rotating by the fire, the fire is just a light up board.
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.
Visible crew/equipment: When Baldrick is called upon to demonstrate the type of clothing required when visiting France, the edge of the scenery can be seen behind him on his right in the area he enters from.
Visible crew/equipment: In the first shot of the theatre play, you can see the boom mike quickly entering the picture to the left on the screen. (00:01:40)
Factual error: At Ms. Miggins' when she says, "There's ever so much hard work," you can see her wearing a bra under her corset. Bras didn't exist at that time. (00:06:35)
Revealing mistake: After Blackadder accidentally kills Smedly with one of the suicide pills, he starts ranting about how useless Smedly is as a hero, because he was dumb enough to fall for the old 'poisoned cup routine.' As he does so, you can see the supposedly dead Smedly behind him, still breathing normally.
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.
Continuity mistake: When the Shadow shoots at Baldrick to get rid of him, she is aiming over his head but the shot lands at his feet.
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: After the Shadow has left Blackadder a captive, she says how much she hates squirrels, and fires two shots from a single shot pistol.
Revealing mistake: When Blackadder lifts the huge turnip it looks really light. A turnip that size would weight a fair bit.
Revealing mistake: The bomb that is thrown at the Prince explodes before the lit fuse actually burns down far enough. (00:03:15)
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: According to the IMDB.com's quote page for Blackadder The Third, it's "Aahhhhh. Hot potato, orchestra stalls, Puck will make amends. Owwwwww."
Gary O'Reilly