Plot hole: In 'The Foretelling', it is said that Richard IV reigned for "13 glorious years" with a caption showing 1485-1498. 'The Black Seal' starts on St. Junipers' Day, 1498, but Blackadder spends 12 months locked up with Mad Gerald. So when the Royal Family are finally murdered it is actually 1499.
Continuity mistake: When McAngus empties his bag on the table and the head falls on it we see the back of the head is turned against the camera. When it cuts to a close up of the head the head is facing in the direction of the previous camera take. When it cuts again the back of the head is turned against the camera.
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.
Visible crew/equipment: After King Richard has been crowned, Blackadder, Baldrick and Percy return to their chambers and start cheering. When Blackadder says "We're safe, and I am a prince of the realm" you can see the boom mike move up in the right corner of the screen.
Visible crew/equipment: When Blackadder returns to the castle after rounding the sheep, Prince Harry says "Ah, Edmund." Blackadder then walks behind one side of the wall. The shadow of a boom pole can be seen on the right side of the wall as he does this.
Continuity mistake: After Blackadder talks to Baldrick and Percy about the plan, Blackadder walks to the window to notice MacAngus getting on a horse to exit the castle. The shot where Blackadder walks over to the window shows only one window above the window Blackadder looks out of and nothing more. A couple of shots later when the camera is in front of Blackadder, two extra windows have appeared from nowhere. There was an iron maiden there which could have blocked the view, but at least six inches of wall are shown between the iron maiden and window, so the window couldn't have been blocked by it. Not to mention the window panes are in a grid of 3x3 from inside, but 4x4 from outside.