Pressurized cabins were first introduced on commercial aircraft with the British de Havilland Comet in 1949. The pressurized cabin allowed passengers and crew for the first time to fly at higher, more efficient altitudes while reducing the effects of altitude sickness and migraine.

Unfortunately, in 1954 two of these aircraft broke apart in-flight, killing all those on board.

A lengthy investigation was conducted by the British Royal Aircraft Establishment and later a report published titled Civil Aircraft Accident Report of the Court of Inquiry into the Accidents to Comet G-ALYP on 10 January 1954 and Comet G-ALYY on 8 April 1954.  The public inquiry concluded as follows at pages 20 and 22:

It is my opinion that the fundamental cause of the failure of the cabin structure was that there exists around the corners of the windows and other cut-outs a level of stress higher than is consistent with a long life of the cabin…
…

As I have already indicated and for the reasons I have given I have accepted the main conclusion of the Report that the cause of the accident to [the aircraft] was the structural failure of the pressure cabin brought about by fatigue.

The investigation revealed the probable cause of these crashes was the advanced metal fatigue found near the corners of the aircraft’s forward square escape hatch windows.

The concentration of metal fatigue near the corners of the square windows was a result of air-frame stress brought on by the pressurized cabin not being evenly distributed across the whole of the fuselage.  Instead, much of the stress became concentrated in the corners of the square windows, resulting in accelerated metal fatigue and ultimately failure.

The later Comet 2 featured a fuselage with a slightly thicker skin and rounded windows.  The rounded windows allowed for safer distribution of metal fatigue caused by a pressurized cabin and are still used in aircraft today.