The Larry Crowne star - who is one of Letterman's favorite celebrities -gasped after she realized he had given the crowd the two-fingered salute.
It happened after Letterman explained to the Oscar winner how his biggest sadness in life was that his son did not have a brother or sister.
Roberts, 43, has three kids with her husband Danny Moder, twins Hazel and Phinnaeus (both 6) plus a son Henry, 4.
Letterman, 64, has a son Harry, 8, with his wife Regina Lasko.
Several years ago Letterman infamously did an inverted single flip-off to a viewer who sent him a letter claiming he watched the show upside down so that blood could rush to his head.
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