The Contradiction sketch from "Monty Python's Previous Record"
 
Host (John Cleese):  With me now is Norman St. John Polevaulter, who for the
                     last few years has been contradicting people.  St. John
                     Polevaulter, why do you contradict people?
Polevaulter (Graham Chapman): I don't!
Host:        But you... you told me that you did.
Polevaulter: I most certainly did not!
Host:        Oh. I see. I'll start again.
Polevaulter: No you won't!
Host:        Ssh!  I understand you don't contradict people.
Polevaulter: Yes I do!
Host:        And when didn't you start contradicting them?
Polevaulter: I did! In 1952!
Host:        1952.
Polevaulter: 1947!
Host:        23 years ago.
Polevaulter: No!
 
<GONG>