'David Letterman's off the cuff remarks might have damaged his career.' Say journalists.
During an interview on the Late Show between David Letterman and Ashton Kutcher. David Letterman's absence of humour was instantly apparent when Ashton Kutch said, "That is not funny." To which David was slow to respond.
David Letterman, shocked, paused for a moment before drinking from his cup that he uses as an excuse to think of a come back. After taking an extra long 'drink' and putting down the mug, David Letterman responded saying, "I am not funny." To which the audience were silent for a moment, before the Teleprompter told them to laugh.
In a later interview; David Letterman admitted that he has never actually been funny, but that he has a funny shaped head that he claims is the reason for his success in comedy.