Factual error: In Blackadder's Christmas Carol, during the Elizabethan part of the 1988 Christmas special, Queen Elizabeth asks Edmund if he has any last words before she chops off his head and sticks it on the Christmas tree. Christmas trees weren't introduced to England until Prince Albert (husband of Queen Victoria) brought it from his homeland in 1840.
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.