There's at least 1 million root causes of the incident and while I have my own predictions, this industry is a great example of checking all possibilities befor

There's at least 1 million root causes of the incident and while I have my own predictions, this industry is a great example of checking all possibilities before jumping to "2 possible causes". It's really unlikely that washers are a root cause (spring/split washers are useless and are definitely not used for this type of apllication). "Correct torque" is not trivial to determine and maybe it was calculated right or wrong based on right or wrong materials/conditions or maybe conditions were changed or maybe... I am 99.9% sure a procedure was followed and a torque or other measurement applied and was documented. This is probably not a trivial error but a cascading failure in design and/or documention and/or integration.