Continuity mistake: At the very end of the episode, when it shows you all the dead bodies, you can see Queenie to the right for a few seconds. The dagger she is holding is pointing upwards. Then there is a close-up of Queenie, who then talks back to you in Prince Ludwig's voice, and the dagger she is holding is now pointing downwards.
Continuity mistake: "Blackadder Back and Fourth": Blackadder and his guests are going down stairs. Blackadder is first, George is second, Melchett is third, Elizabeth is fourth and Darling is fifth. Blackadder gives his notebook to George. When they arrive downstairs, Melchett and Elizabeth are ahead of George. This is not logical, since the stairs are too narrow and they could not have passed George.
Continuity mistake: When Blackadder and the other soldiers are about to go "over the top", you can see two other soldiers standing behind Darling. They are presumably in the upper part of the trench. When Blackadder blows his whistle, you see all the soldiers climb onto their ladders and then start to rush over, side-by-side. In the next shot, when we see the soldiers emerging from the trench in slow motion, the other two soldiers are still back in the trench waiting for Darling to climb out before they move, as the two shots were filmed in two different studios.
Continuity mistake: When Blackadder, Baldrick and George are out in the no man's land, Blackadder screams and Baldrick asks why. Blackadder answers he put his elbow in a blob of ice cream while looking at Baldrick. When it cuts he is looking straight forward instead.
Visible crew/equipment: When Harry tells Blackadder to be in charge of the welcoming of King Richard's return, as Blackadder turns around and walks against the fireplace saying "12 months of chasing sheep.", if you look closely on the top of the fireplace you should be able to see the shadow of the boom mic moving. It's quite hard to see, but it's there.