Apollo 11 Spaceflight, First Manned Moon Landing

On July 20, 1969 the lunar module (LM) Eagle separated from the command module Columbia. Collins, alone aboard Columbia, inspected Eagle as it pirouetted before him to ensure the craft was not damaged.
As the descent began, Armstrong and Aldrin found that they were passing landmarks on the surface 4 seconds early and reported that they were "long": they would land miles west of their target point.
Five minutes into the descent burn, and 6,000 feet (1,800 m) above the surface of the Moon, the LM navigation and guidance computer distracted the crew with the first of several unexpected "1202" and "1201" program alarms. Inside Mission Control Center in Houston, Texas, computer engineer Jack Garman told guidance officer Steve Bales it was safe to continue the descent and this was relayed to the crew. The program alarms indicated "executive overflows", meaning the guidance computer could not complete all of its tasks in real time and had to postpone some of them.
In a letter to Datamation, March 1, 1971, Margaret H. Hamilton, Director of Apollo Flight Computer Programming MIT Draper Laboratory, Cambridge, Massachusetts, stated: “Due to an error in the checklist manual, the rendezvous radar switch was placed in the wrong position. This caused it to send erroneous signals to the computer. The result was that the computer was being asked to perform all of its normal functions for landing while receiving an extra load of spurious data which used up 15% of its time. The computer (or rather the software in it) was smart enough to recognize that it was being asked to perform more tasks than it should be performing. It then sent out an alarm, which meant to the astronaut, I'm overloaded with more tasks than I should be doing at this time and I'm going to keep only the more important tasks; i.e., the ones needed for landing...Actually, the computer was programmed to do more than recognize error conditions. A complete set of recovery programs was incorporated into the software. The software's action, in this case, was to eliminate lower priority tasks and re-establish the more important ones...If the computer hadn't recognized this problem and taken recovery action, I doubt if Apollo 11 would have been the successful moon landing it was”.