It was a moment that seemingly called for a follow-up question.

On “Good Morning America” on ABC Friday, the former New York City mayor, Rudolph W. Giuliani, stated that “we had no domestic attacks under Bush; we’ve had one under Obama.”

The misstatement  which omitted the Sept. 11, 2001, attacks, which occurred nearly eight months after President Bush took office  was roundly criticized Friday, even by some of ABC’s correspondents. “Even if you assume he meant post-9/11 this is hard to justify,” Rick Klein, the primary author of ABC’s online political memo The Note, wrote on Twitter.

But at the time, the interviewer, George Stephanopoulos, let Mr. Giuliani’s assertion go unquestioned. The segment ended a minute later with a joke about the day’s snowfall in New York City.

Amid an online outcry, Mr. Stephanopoulos, a veteran political anchor who became a co-host of the show last month, acknowledged later Friday that the lack of follow-up was a mistake.