In the first play there are two daggers and two swords on the stage when the actors are taking their applause. A moment later, when the anarchist jumps onto the stage, both swords and a dagger have gone.
When Keanrick enters the Prince's house, he misquotes a line from Macbeth, saying "Lead on, Macduff", rather than "Lay on , MacDuff". This is a common misquotation, and may been used to indicate the poor professional capabilities of Mossop and Keanrick.
When reading the job vacancies, Edmund says that George Stephenson 'has invented a moving kettle'. It was actually Richard Trevithick that invented the steam locomotive, not Stephenson.
When Blackadder and George are at the theatre watching "Julius Caesar", electric lights can clearly be seen along the floor at the front of the stage.