Visible crew/equipment: When Baldrick is called upon to demonstrate the type of clothing required when visiting France, the edge of the scenery can be seen behind him on his right in the area he enters from.
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.
Answer: It's not known what Baldrick's first name is. When he was a kid and introduced himself as Baldrick to some other kids, they responded with, "We know. Sod off, Baldrick." Sod off is another way of telling someone to go away or get lost. So when the kids said "Sod off, Baldrick," what they were really saying to him was, "Get lost, Baldrick."