For his first year of deer hunting I lent a friend a rifle. I had hunted with this rifle for years and it had never failed me in the woods or at the range, but now I was using a muzzleloader so it saw little use. We tested the rifle at the range before the season, got it sighted in for him, did some practice, and again it worked fine.
Fast forward to a cold drizzly day. He's sitting in a brush blind, getting rained on, and a nice deer comes out. He aims, pulls the trigger, and CLICK! The deer hears the click and is now staring at him from about 60 yards away. He doesn't know what to do so he racked the bolt and that sent the deer running off. I get a frantic series of texts. He assumes that it must be user error because he's new to firearms and hunting.
A couple hours later another decent deer comes out. As he's sizing it up in the scope, a better deer walks up behind the blind and blows from just a few yards away. That puts the deer that he's aiming at on alert. He paused for a moment, unsure what to do. Should he turn and try for the one behind him and risk scaring both away? He decides to go for the one he's already aimed on and pulls the trigger... CLICK!! That sends the deer behind him running and spooks the one he tried to shoot as well. Both gone. I get the most dejected phone call ever.
I meet up with him. We keep the gun out in the cold, on the back of the truck, and drive to a range to test fire the gun. It fires every time we pull the trigger. The ammo is high quality premium stuff that's never given me any trouble in the past. Then I notice that the indent on the primer is less pronounced on the two rounds that failed to detonate compared to the shells we are ejecting at the range. The two failed shells both fire when we try them again.
Best I can figure, the gun might have had grease, dirt, or gunk accumulated in the channel that the firing pin pops out of. Combine that with the cold wet conditions and there must have been just enough friction to slow the speed and force of the firing pin enough that the primer wasn't being hit hard enough to detonate. We tried to remedy the potential issue by putting a drop of oil on the opening of the firing pin channel and dry firing the rifle a bunch of times (with snap cap dummy rounds), then repeated that many times. The gun has never again failed to fire but I have lost confidence in that rifle.