A few years ago I was night hunting hogs with my 7mm magnum BAR and I was up to bat when a huge boar hog walked into the plot. I took careful aim at 50 yards and click, nothing happened. Fortunately I was hunting with friends in my shoot house and one of the other guys killed the hog. I was frustrated to say the least. I took it to a gunsmith who called the next day and said he had never seen it before but a cleaning patch from a previous cleaning had worked its way into the internal mechanism and kept the gun from going fully into battery. I had made many one shot kills with that rifle and never dreamed it would not fire. Easy fix but a great object lesson.
We often inspected officers' firearms and found enough missing or loose screws, empty revolvers, no magazine, and a few times an empty holsters to make us contiinue inspections.