It doesn't happen often, but when your optics go awry, you need to act decisively.
Scope problems in the field can be insidious, and if you suspect you have a problem and can't isolate it, sometimes you're better off borrowing another rifle.
Over the years I've had quite a few scope failures on the range, either bad scopes right out of the box, or scopes that just wore out and gave up the ghost. But it had never happened to me in the field. This has purely been luck of the draw, a numbers game, since I do a whole lot more shooting on the range than I do in the field.
But it can happen. Since both Murphy's Law ("Whatever can go wrong will go wrong.") and First Corollary to Murphy's Law (". . . at the worst possible time.") apply, it's amazing it hasn't happened to me before. I have had scope mounts break, and I've had mount screws shear, and I've had iron sights bend and twice, I've had rear iron sights fall off a rifle. But until now my scope failures have taken place on the range.
So I've been lucky. Riflescopes, like the rifles we put them on, are mechanical devices and subject to failure every now and then. So, while the odds of it happening at a given moment are low, and are mitigated by sticking with quality brands, it can happen to any scope at any time, and over the years I have seen examples of virtually all makes of scopes fail dismally.
Since scope failure is totally random it isn't something you can guard against, but you must keep the possibility in the back of your mind, because it can be extremely insidious. If you're lucky the failure will be catastrophic--the reticle falls out, or the front objective falls off--because then you know you have a scope problem.
But what if the scope shakes just a wee bit loose inside or the scope mount is just a wee bit loose? The rifle is shifting zero or won't quite hold zero, but the problem isn't so gross that you suspect the scope or mounts as the culprit right away.
Now you can do some damage. On the range you might abandon perfectly good loads, recrown a perfectly good barrel, or, perhaps worst of all, hack on perfectly good bedding, when all that was really needed was to put a different scope on the rifle and send the offending scope back for warranty repair.
In the field, you can wind up missing an animal and kicking yourself for years when it may not have been your fault at all. And of course, since Mr. Murphy is lurking out there, the animal you miss will have been the trophy you had been seeking for decades.
The thing is, scope problems are not uncommon. In my business of frequently testing rifles for various articles I'm often throwing unfamiliar scopes on unfamiliar rifles. There have been too many occasions when I've had a rifle that ought to shoot better than it did--and a "slightly bad" scope proved to be the problem.
However, field shooting is not a perfect science. We are usually shooting from less than perfect positions, not always at stationary animals offering crystal-clear presentations, and not always at known distances.
I'll be honest: I breathe a sigh of relief when I make a decent hit on any game animal, and if I'm an inch or two off from a perfect hit I don't second-guess myself too much. So you can have a situation where you have failure masked by success.
This is exactly what I had going on a recent hunt in Argentina. I was shooting my 7x57, which was shooting perfectly at the range, but when I arrived in Argentina the bullet strike was inexplicably high, so I adjusted the scope.
I shot a water buffalo at maybe 30 yards with a 175-grain Barnes Super Solid, a perfect brain shot. No problem. Hunting with hounds, I shot a capybara at about 10 yards, and I killed a monstrous wild pig at maybe 10 feet. No problem.
I also missed a collared peccary running across a narrow opening, a fast shot at 50 yards. A tough running shot that looked good but wasn't. No problem. But since we were also going to hunt free-range blackbuck antelope I thought I'd better check my zero. I fired a shot at a 50-yard target and discovered it now was a bit low, so I adjusted the scope.
The next morning I missed another shot at a collared peccary, almost the same situation--moving fast across a narrow opening. Had I connected it would have been a fantastic shot, no shame in missing, nothing lost in trying--except the crosshairs had looked really good when the rifle went off.
I headed to the range and hung a target at 100 yards. The rifle was now left, right, up and down and was holding an eight-inch group.
The screws were all tight, humidity hadn't changed, but a very good scope, never mind which brand, has suddenly and mysteriously gone south. I don't much care about missing a javelina, but if I hadn't gotten suspicious I wouldn't have had a snowball's chance of taking what turned out to be a great blackbuck antelope with a borrowed .270.
These things shouldn't happen, but they can and they do. There is no way to properly guard against them, simply because any mechanical device can fail. However, what you can do is follow your instincts.
If you have even the slightest suspicion of a problem, chase it down and try to isolate it. Most of the time the problem will be the shooter, whether it's you or me--but not always--and when a scope goes out you have no choice but to switch it out or borrow a rifle.