Revealing mistake: When Percy is holding the 'Green' in his hands, light of the green reflects on the wall behind Blackadder. However the way Percy is holding the 'green' in front of Blackadder, Blackadder's shadow should be on the wall behind him, but it isn't. It's obvious green lighting is used to light up the back wall.
Revealing mistake: After Blackadder accidentally kills Smedly with one of the suicide pills, he starts ranting about how useless Smedly is as a hero, because he was dumb enough to fall for the old 'poisoned cup routine.' As he does so, you can see the supposedly dead Smedly behind him, still breathing normally.