The De Havilland Comet. The first commercial jet airliner in history. Designed and built in Great Britain. It began operating in 1952. Shortly after its launch, three planes crashed after their fuselages disintegrated in mid-air. Do you know why? The square windows. The aircraft flew 10,000 meters, the same operational ceiling as today's jets and therefore had to be pressurized and depressurized repeatedly. This caused cycles of contraction and expansion of the fuselage. At the angles of the square windows, this caused material fatigue and cracks. After several cycles, the fuselage would crack in mid-air. This fatigue was not understood at the time, and therefore De Havilland paid the price for being a pioneer. De Havilland discovered the cause of the problem and redesigned the aircraft windows. The photo below shows a model built after the investigation. To this day, all commercial or executive jets have rounded windows for this reason.