On Southwest 1380, initial indications are that a fragment of the engine blade pierced the engine casing, which then hit the airplane window (or perhaps a part of the casing did). According to the BBC,
Mr Sumwalt also said a casing on the engine was meant to contain any parts that come loose but, due to the speed at impact, the metal was able to penetrate the shell.
Is this an reasonably expected containment failure? Or is the failure to contain surprising to industry experts?