Percy is shown wearing a beard, despite having shaved it off in the previous episode.
Nursie refers to the ointment used when the Queen's sister Mary was beheaded. In fact, Elizabeth I's half sister Mary I was not beheaded.She died quite young but from natural causes. The writer is probably thinking of Mary Queen of Scots who was beheaded but wasn't Elizaberh's sister.
Sir Francis Drake was not executed on the orders of Queen Elizabeth II. He died of dysentery after attacking San Juan in Puerto Rico.
Queen Elizabeth mentions that there are hundreds of Catholics that want to be beheaded. Queen Elizabeth was relatively tolerant and repealed heresy laws, but she said that they "desperately want" to be beheaded (instead of "need"), so the mention of Catholic execution was not religious intolerance, Elizabeth just wanted to accede to the Catholics' demands.
When Edmund is reading a death warrant he holds it horizontally, but you can see that the text is written the other way around.
Edmund Blackadder is seen sitting in a revolving chair. The swivel chair was invented by Thomas Jefferson in the 1700s.
Elizabethan Edmund refers to calling the "police" but they would not exist in Britain until the 19th century.
Blackadder refer to handing over his enemies to the "police". They wouldn't exist in Britain until the 1800s.
If Blackadder heard Percy saying "How do we know you're not a gloater?," then Lady Farrow surely would've heard Blackadder, Percy, Ploppy, and Baldrick discussing what Lord Farrow sounded like and how tall he was, which would be a dead giveaway.
At the end, Edmund, for a second time is trying to disguise himself as a prisoner on death row but unlike Farrow, Ponsonby hasn't been pardoned and the Queen even makes reference to "having him killed" before going to see him. Edmund could simply invent a plausible reason why he's been executed earlier than scheduled.
Nursie refers to Elizabeth's sister Mary (Mary I of England) having her head chopped off - in fact it was her cousin, Mary Queen of Scots, whom Elizabeth had executed. Mary I died of natural causes, with Elizabeth succeeding her.