System integration issues in Apollo 11

“Houston, Tranquility Base here. The Eagle has landed.” Two obscure errors almost prevented these words from being spoken. The errors were not made by the crew of Apollo 11 or by the controllers in Houston, nor were they made during the mission. Rather, they were made by engineers and managers years before the flight. How they happened, and how they went substantially undetected and effectively ignored, is a pair of lessons in system integration that avionics engineers must never forget.