Those are not the only options. * Improper bolt tensioning process where bolt tension is later unintentionally reduced due to tensioning order. * Design or ma

Those are not the only options. * Improper bolt tensioning process where bolt tension is later unintentionally reduced due to tensioning order. * Design or material failure where bolt tension post-installation drops more than design allows. * Miscalibrated or otherwise defective equipment. * Other failure in manufacturing not caused by the worker, e.g. missed a necessary second or third tensioning pass (different torque or rotation angle) without appropriate failsafes or final validation to discover it. Bolt tension validation is normally done with an ultra-sonic scan. Torque only provides a rough approximation of the tension at time of installation, and is only suitable where precision is not a requirement. I suggest https://www.youtube.com/watch?v=XLzTB4KLCxU https://www.youtube.com/watch?v=XLzTB4KLCxU for those curious about bolts - a very specific, but interesting video.