Monty Python and the Quest for the Holy Grail - The Dreaded Word
When King Arthur returns with a shrubbery for the Knights who Say Ni, the leader (Michael Palin) comments that "It is a good shrubbery." Later we learn that the word "it" causes the Knights to scream in pain.
Palin actually says it twice in this segment, but I can't remember the exact text of the second reference.
on 07-21-2000 and