It's tricky to invoke 9/11 on a debate stage. Trickier still when the question has nothing at all to do with national security.

Bernie Sanders slammed Hillary Clinton's ties to Wall Street, stating that her plans to reform the financial industry are "not good enough."

Why, Sanders asked, has Wall Street been a "major contributor" to Clinton's campaign? "Maybe they're dumb and don't know what they're going to get, but  I don't think so," he said.

Clinton defended herself.

"He has basically used his answer to impugn my integrity, let's be frank here," Clinton said, noting she has "hundreds of thousands" of donors, many of them small.

And then: Sept. 11.

"I represented New York and I represented New York on 9/11 when we were attacked. Where we were attacked? We were attacked in downtown Manhattan where Wall Street is," she said.

"I did spend a whole lot of time and effort helping them rebuild; that was good for New York, that was good for the economy and it was a way to rebuke the terrorists who had attacked our country. It's fine for you to say what you're going to say but ... my proposal is tougher, more effective and more comprehensive because I go after all of Wall Street, not just the big banks," she said.

Twitter was incredulous.

And opposing campaigns pounced.

Moderator Nancy Cordes read one of the Twitter responses to Clinton. Was she sorry she'd brought up the attacks?

“I'm sorry that whoever tweeted that had that impression because I worked closely with New Yorkers after 9/11 for my entire first term to rebuild, so yes, I did know people. I've had a lot of folks who give me donations from all kinds of backgrounds say, 'I don't agree with you on everything but I like what you do, I like how you stand up, I'm going to support you," Clinton said.