After years and years of mocking the sordid sex lives of other celebrities, David Letterman found himself on the other side of the joke in 2009 when it was revealed he was having sexual relations with female members of his staff.

Generally speaking, it's impolite for guests on his show to bring this up. But Jon Stewart did, although the faux pas was completely inadvertently.

It started off with a joke about Mitt Romney, who Letterman said looks like the guy on the package of underpants. Stewart agreed, and in the riffing that followed he said to Letterman "I would think at this stage of your career, you have people that could just sit there and hug your groin for you...You could have like a group of people . . ."

If the crowd's increased laughter didn't give Stewart a hint that he had wadded into dangerous waters, Dave's gentle scolding of him did. ""I had a little trouble along those lines," Letterman reminded Stewart.

"I'm halfway through that bit and all the sudden I realize, oh what the [bleep] am I doing," a flustered Stewart admitted. It'was all in good fun though, at least on camera. Hopefully this won't kick off another late-night war.