Remember when the astronauts accidentally entered a code which totally reset the computer's location data, and had to guide it around the moon basically from scratch?
If you're talking about this first landing, it wasn't an accident. It was due to a hardware bug that wasn't discovered in testing. What had happened is the
rendezvous radar (which was meant to help locate the command module on the trip back up but wasn't needed for descent) was still feeding data to the computer when they were in descent. The unnecessary data was just saying "I don't see anything, I don't see anything, I don't see anything..." Well, the computer still had to process that data and when added to the data that was being processed for the descent was enough to put the processor load very close to its limit. Then, when Buzz wanted to get more details about their descent parameters, he typed a code into the computer that instructed the computer to provide some additional landing data. This was not an accident, it was something he had done many times in training with no problems. But this time, due to the computer already being near its max processing capability, the computer experienced the equivalent of the "blue screen of death" (it basically ran out of memory) and had to be restarted. There were 5 codes all together: Three of them happened when they were still relatively high above the moon and were precipitated when Buzz entered the code which resulted the computer memory overload. Buzz made two more attempts after that first alarm, but after the third alarm, he put two and two together and realized that these alarms seemed to be tied to the code he was typing in (remember, he had done this hundreds of times in training with no issues).
But then, when they got much closer to the surface, they entered a new phase of the descent in which the computer was under even more load without any additional inputs from Buzz. This extra processing in this latter phase, when combined with the unnecessary data from the ascent radar, caused the computer to get overloaded without any extra inputs from Buzz and they got two more alarms and had to do two more restarts - this time much closer to the surface. (Had NASA designed the system to shut off or disconnect the rendezvous radar during descent, none of these alarms would have ever happened). As they got closer to the surface, Neil, while looking out of the window, did not like where the indicator was showing him they were going to land, so he typed in a code to enter the final phase of the landing sequence a little early. In this final phase, he was able to move a dot in the window indicator (kind of like a heads up) to tell the computer his new desired landing location. He wasn't really "steering" the craft as popular legend would tell us, but just moving a dot on his window screen telling the computer - don't land there, land here." Once that was done the computer took over and maneuvered the craft to land at the desired spot. This final phase was something that was part of the planned sequence, Neil just entered it a little early. He had no idea it was going to stop the alarms, he only did it because he didn't like the landing spot that they were heading for.
If you
jump ahead to about 47:53 in the video, he does a great job explaining exactly what happened and why they got those codes in much better detail. The above is just a summary.
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Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
