It's really hard to land on the moon....

[Radar]

The inoperative rangefinder was, per a NASA engineer I spoke with, caused by someone forgetting to remove the hardware interlock that prevented the laser from emitting. This is almost as bad as crashing a probe into mars because someone ignored the documentation and used the wrong units. Fortunately, they had an experimental backup, so they could still land.

There were many other failures like this. For example, Genesis probe that gathered solar wind particles very close to the Sun crashed full-speed into Earth because the parachutes did not open. The reason? All four, independent accelerometers were mounted upside down.