"The man has a lot of dates to keep track of," one aide told reporters. "For one thing, he's had three different wedding anniversaries."

Rudy Giuliani

The former New York mayor had been cruising through the town hall meeting up to that point, answering a wide range of questions about immigration, the economy, and global warming, all by referring to 9/11.

But the tone of the event changed abruptly when one audience member, Tracy Klujian of Cedar Rapids, asked the GOP frontrunner, "Can you name one thing that happened on September 12?"

Mr. Giuliani seemed taken aback by the question, clearing his throat and drinking from a glass of water as if to buy time before responding.

"That's a good question," Mr. Giuliani said. "September 12 happened one day after September 11 -- and we must never forget the lessons of September 11."