Corrected entry: Both Captain Blackadder and Captain Darling make references to facetious conversations with Pope Gregory IX. Obviously they didn't speak to the pontiff, but it is not even a joke that way because Gregory IX was pope from 1227-1241; if they wanted the Pope reigning at the time of the Great War, they meant to say Benedict XV (1914-1922).
Corrected entry: The episode "Bells" and the episode "Head" appear to be inversed in their order - Lord Percy has a beard at the beginning of "Bells," and shaves it off. However, in "Head," he's got it back. In all of the other episodes, he has no beard. Also, the song at the end of "Head" mentions Lord Blackadder's origins (it mentions Edmund's great-grandfather), which makes it more a follow-on from the 1st series.
Corrected entry: Queen Elizabeth asks Percy "play awhile, to calm my spirits" and he starts singing clapping games and nursery rhymes. One of these is "Ring o' ring o' roses", a song which was not written until the late 1800's (and which, contrary to urban legend, has nothing to do with the bubonic plague - see http://www.snopes.com/language/literary/rosie.htm for more detail).
Corrected entry: While it is conceivable that Flashheart would be so lost as to be unaware of which side of the lines he crashed on, it is absurd that he would mistake a uniformed British officer for a German. The uniforms are completely different - they aren't even the same colour. 'Friend or foe' recognition was hammered into pilots at every stage of their training to prevent them accidentally bombing or strafing their own side.
Corrected entry: In the episode "Beer", set in the 1560s, the "Happy Birthday" song is sung by several characters. This song was not written until 1893 by two American school teachers, copyrighted in 1935. Neither is the song in the public domain. It still earns significant fees to the copyright holders each time it is recorded for sale, i.e. in a movie or on television. In most of the episodes, the various historical inaccuracies are intentional. This one, though, appears to have been an accident, a genuine mistake on the part of the writers. It should be noted that unintentional historical errors are very rare in Blackadder, "unintentional" being the key word.
Corrected entry: Nursey says that ointment is what's needed when your head's been cut off and that's what she used with Queenie's "sister Mary when they done her." Mary, Queen of Scots, was the only close relative of Elizabeth that was beheaded and called Mary but she was a cousin. Elizabeth's half-sister Mary was not beheaded, she was queen (as Mary I) before Elizabeth and died of influenza and cancer. Also, Mary was beheaded in 1587 when Elizabeth was 54, Miranda Richardson hardly seems 54 and in "Potato," Sir Walter Raleigh has just returned from America and mentions that he left Plymouth in spring of 1552 so Mary is over thirty years away from her beheading.
Corrected entry: I know Percy's not that bright, but he adds "Cardinal Wolsey" to the list of party guests. Given that Wolsey died in 1530, and Queen Elizabeth ruled from 1558 onwards (she wasn't even born until 1533), he must have been dead for at least 28 years by this point (even in the Blackadder world of history). Obviously it's done for the joke, but it's still wrong.
Corrected entry: In the first series the lead character's name was written as two words, "The Black Adder" (well, technically three words), and in subsequent series it was written as one word, "Blackadder". This is not a mistake, however; this is the normal way that languages evolve. The name evolved from the two word pseudonym used by Prince Edmund into the one word name used by his descendants.
Corrected entry: In 'Blackadder back and Forth', we hear George ask Blackadder for a pair of old underpants so that he'll pay him the £10,000. However, after Blackadder returns from the past the first (and second) times, he has not got the underpants, so why is George giving him the money?
Corrected entry: In the opening sequence, watch the members of the band who aren't in the main cast - there are at least two shots where one or more of the band are obviously smiling/laughing at what the main cast members are doing at the time (most noticeably is when Blackadder yells the "Eyes Right" command - there's a guy a short distance behind Blackadder who grins during the salute.