The first commercial jet airliner crashed due to a hidden design flaw

The de Havilland Comet, launched in 1952, was the world's first commercial jet airliner and a triumph of British engineering. Three planes disintegrated in flight in 1953–1954, killing all on board. Investigators at Farnborough ran the most extensive aircraft forensics ever attempted, submerging a fuselage in a water tank and pressurising it repeatedly. They found that the square windows acted as stress concentrators — metal fatigue cracked from the corners. Every subsequent aircraft design uses oval windows specifically because of the Comet disasters.

Source: Withey, Comet! The World's First Jet Airliner (1999)