If there have been the only ones I have heard about are the ones we are responsible for. Of course we do most of the close air support, that tends to put us in the greatest chance of having it be our fault.
Also had the question over whether there was the opposite situation (with coalition forces killing Americans), but—as Abathar pointed out—the odds are stacked against the Americans (for doing the killing) because they seem to be the primary ones in the air, which seems to be where a lot of these friendly fire incidents are coming from.