“On the other hand, what this probably is is simply someone or probably two people having a careless moment.”
If these people have a careless moment they die. They’re VERY careful.
So are bomb disposal technicians and yet they occasionally blow themselves up.
All workers make mistakes occasionally. Most of these mistakes are benign. Many go totally unnoticed even by the people that made them. Some of the mistakes will be corrected by the person on the spot or by the next worker to come by.
But occasionally a mistake is made and not caught. That mistake may sit there for days or weeks unnoticed. Then another worker comes along and does something correct or incorrect that brings that mistake to light in a bad way.
It is a fact of life in the human world. People make mistakes; all of us everywhere. The best you can do is teach people techniques to minimize the number of mistakes.