laurentian
Regular
User error/carelessness is mostly the problem.
I'd be interested to know what the source is for that claim. (I'm talking captive bolt systems here).
"User error" is a difficult thing to define when there's a live animal that could be moving around "on the other end" but even so, in my experience, this is far from a regular cause of incomplete stunning. A captive bolt gun failing to work first time will cause no real harm to the animal. The issue would be if the gun worked but did not hit the "right spot". People who use these things in approved establishments are (or should be!) trained and licensed professionals.