Revealing mistake: When Baldrick is on the metal spit rotating by the fire, the fire is just a light up board.
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.
Revealing mistake: When Blackadder lifts the huge turnip it looks really light. A turnip that size would weight a fair bit.
Revealing mistake: The bomb that is thrown at the Prince explodes before the lit fuse actually burns down far enough. (00:03:15)
Answer: According to the IMDB.com's quote page for Blackadder The Third, it's "Aahhhhh. Hot potato, orchestra stalls, Puck will make amends. Owwwwww."