From the Huffington Post, September 9th, 2009:

During President Obama's major health care speech on Wednesday, South Carolina Rep. Joe Wilson (R) yelled out "You lie!" when the President said the legislation would not mandate coverage for undocumented immigrants.

The Associated Press noted:

It wasn't the only interruption during Obama's speech to a joint session of Congress in the House of Representatives. Earlier, Republicans laughed when Obama acknowledged that there are still significant details to be worked out before a health overhaul can be passed.

Wilson's outburst caused Obama to pause briefly before he went on with his speech. Overhead in the visitors' gallery, first Lady Michelle Obama shook her head from side to side.

During an appearance on CNN after Obama's speech, Sen. John McCain (R-Ariz.) said Wilson's outburst was "totally disrespectful -- [there's] no place for it in that setting or any other and he should apologize immediately."