Before you shoot, or before you get ready to employ your weapons systems you need to check them. At the range, we had a few people who found their rifles would not work because the adjustable gas system were in the wrong setting. The worst case was when one new POF AR15 rifle owner found his rifle was only functioning as a single shot. When I looked at his rifle, I found the gas plug was gone. Fortunately his friend found the plug downrange. Once it was installed and placed in the proper setting, his rifle ran great.
Aside from gas systems, it is also often that optics will be mounted loosely. Sometimes batteries are dead in the optics. Other times a firearm with no lubrication will cause all sorts of issues during a range session.
So, before you go to the range, or when your doing your regular checks of your fighting weapons, make sure to check the little details like if your optics and lights work, that your sights and gas systems are set correctly, etc. As it is said, “Proper Planning Prevents Piss Poor Performance”, and “Complacency Kills”.