Apparently David Letterman is upset about Obama's lies regarding Romney's comments about the car industry. Obama misrepresented (consciously) Romney's previous comments about how to deal with the auto-industry crisis a few years back. Romney said they should go through a managed bankruptcy with government backing. But, Obama has stated that Romney just wanted to let them go out of business with no backing, which is not at all what Romney said.
Here is video of Letterman: