According to the mascon entry on wikipedia:
At that time, one of NASA's highest priority "tiger team" projects was to explain why the Lunar Orbiter spacecraft being used to test the accuracy of Project Apollo navigation were experiencing errors in predicted position of ten times the mission specification (2 kilometers instead of 200 meters). This meant that the predicted landing areas were 100 times as large as those being carefully defined for reasons of safety. Lunar orbital effects principally resulting from the strong gravitational perturbations of the MASCONS were ultimately revealed as the cause. William Wollenhaupt and Emil Schiesser of the NASA Manned Spacecraft Center in Houston then worked out the "fix" that was first applied to Apollo 12 and permitted its landing within 163 meters of the target, the previously-landed Surveyor 3 spacecraft.
I've been unable to find any reliable information on the fix mentioned here. What was the fix, exactly? Is there a reliable reference for this information?