The 9/11 comment came in response to Sanders's attacks on her willingness to accept Wall Street donations.
"Where were we attacked? We were attacked in downtown Manhattan where Wall Street is," Clinton said, with a passion that almost suggested that the question was out of line. "I did spend a whole lot of time and effort helping them rebuild."
"That was good for New York. It was good for the economy and it was a way to rebuke the terrorists who had attacked our country," she added.
The answer did not rebut Sanders's point, that Clinton might be beholden to special interests because she has accepted millions from the financial industry. On Twitter and Facebook, the moment became the most-talked-about exchange of the debate - and not to her benefit.
Clinton "vehemently offers support for Wall Street as post-911 recovery effort. Does that fly?" former Obama adviser David Axelrod asked on Twitter.
It didn't help that during the event, a biting follow-up question came from social media. "I've never seen a candidate invoke 9/11 to justify millions of Wall Street donations until now," wrote Andy Grewal, a law professor at the University of Iowa.