Revealing mistake: It is apparent that the door Flash pushes Percy through is fake. The door bends inwards like polystyrene when it should be made out of wood.
Revealing mistake: When Flash says "I've got a plan and it's as hot as my pants", his moustache is seen falling off.
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."
Gary O'Reilly