In the first half of this video President Obama is explaining the absurdity of the so-called death panels that would not exist under his health care plan. In the second half he begins explaining another popular falsehood that the plan will provide health care to illegal immigrants (it won't) at which point Representative Joe Wilson from South Carolina yells, "You lie!" to the dismay and jeers of the entire house.

See CNN's fact check: Will the health care bill really pay for illegal immigrants?

From YouTube: "Sept. 9, 2009 -- It's hard to hear at first, but yes, at 1:28 into this clip, the President of the United States was indeed heckled while addressing Congress. The outburst came from Rep. Joe Wilson of South Carolina, who shouted, "you lie!" as Obama argued that his health care plan would not cover illegal immigrants."