Various nuclear weapon incidents over the years have demonstrated how - even during the most tightly regulated engineering activities - failures by leadership to ensure standards are met and procedures followed actually create more holes in the Swiss cheese making it easier for the holes to align !
The 1984 incident at RAF Bruggen - when a WE177 nuclear weapon - in a transit container because it was being moved between a C-130K and the SSA - fell off a Type SA Trolley being towed by a Land Rover because it was not restrained - was a prime example of correct procedures not being followed - not just on the day in question but going back years.
BOI Narrative of Events here:
https://webarchive.nationalarchives....tInGermany.htm
Again, at RAF Marham in 1988, multiple deviations from the requirements of SD814 - no Weapon Load Supervising Officer on site, Special Weapon Load Team not properly constituted, disoberdience to No Lone Zone procedures, failure to follow loading procedure, unauthorised button pressing - led to a drill WE177 being explosively jettisoned from a Tornado GR1 onto the floor of a HAS. [all procedures for live weapons were supposed to be applied to drill weapons].