Pitt the Younger's age is exaggerated for humour purposes. He was 24 the first time he was Prime Minister.
When delivering his speech to the Commons, Pitt the Younger is reading from cue cards; his eyes never leave the one spot.
William Pitt the Younger claims to have been talking to chancellor Metternich of Austria. Metternich did not become state chancellor of Austria until 1821, 15 years after Pitt's death.
According to Blackadder, William Pitt the Elder is succeeded as Prime Minister by William Pitt the Younger. Pitt the Elder was Prime Minister from 1766 until 1768, while Pitt the Younger did not become Prime Minister until 1783. Both Pitts died before the Regency Era.
Lord Liverpool was Prime Minister during the Regency period (1811-1820), not Pitt the Younger who died in 1806.
When Blackadder calls the number of votes for Baldrick and everyone claps, a boom mic is visible in the left hand corner.
Pitt the younger states "as I said to Chancellor Messernich at the congress of Strasbourg...", this is an error by the actor Simon Osborne. He means to say "Chancellor Metternich" (noted Austrain chancellor Klemens von Metternich).