They happen. Some units do more training than others. Some people get issued a weapon they’re not familiar with. Some people do something stupid. And anyone is capable of making a mistake, especially when they’re sleep deprived. I always associated units that had a keen fetish for clearing barrels as more likely to ND, but that’s subjective bias.
Probably the most common is someone clearing a weapon like they have so many times before that it is an automatic process and, for whatever reason, the sequence in their brain misfires and they do something out of order.
