A negligent discharge is caused by either ignorance or carelessness. The person can be considered ignorant, if they have not been trained. They are careless, if they disregard their training. There is no such thing as an accidental discharge.
I know, that's why I added "hmmm".
However negligent behavior that causes something like this is still accidental, in the sense that it's not deliberate, just negligent. Which of course is the term I should have used, Negligent Discharge.