A lot of times you may see discussions of a weapon that say it should be able to place a warhead within X number of yards of a target. At one time when describig ballistic missiles, and I assume it still is the case, this accuracy was defined by circular error probable (CEP). That is, 50% of your warheads would land within the radius describing your accuracy.
Obviously, most misses would be near the center of the target circle. But not always. Some very small percent will land 100 yards from the launch point, for example.
Another example would be the missiles that Russia fired at targets in Syria from vessels in the Caspian Sea:
As many as four of the 26 long-range cruise missiles that Russia said it fired at Syrian targets landed instead in Iran, two U.S. defense officials said Thursday.
I'm not sure if CEP applies to non-ballistic missiles, but there must be some similar concept for guided weapons.
Russian missiles probably have a wider spread of landing than Western missiles, but even a very small number for CEP accuracy will result in things like this happening.
UPDATE: A reader who wishes to remain unnamed writes that CEP does not apply to guided (as opposed to aimed) weapons. So while guidance can go wrong in a number of areas, the concept doesn't apply to those cruise missiles.
So I guess the only similar concept is that guided weapons miss sometimes, too.
The point remains that just because some of the Russian cruise missiles crashed early doesn't mean we should dismiss them as crud. I assume some of ours have plowed into the dirt (or waves) on the way to targets, too.
UPDATE: Wait. I misread the email. The concept of CEP still applies to guided weapons, but it is quite different.
If guidance is working, the CEP will be pretty small and you can expect the hits to really cluster around the aim point, as my reader wrote. But if the guidance goes bad, the distance outside of CEP where the warhead lands can be dramatically greater compared to aimed but unguided warheads. Let me quote my reader: "The error distribution for guided munitions typically has a nice tight center and then extremely long tails."
So there you go. CEP applies and sometimes precision warheads miss. And that's separate from the issue of whether warheads "miss" because they are mistakenly told to go elsewhere and the warheads go exactly where they were wrongly guided to go.
Actually, I feel better now. When I started writing this post, it occurred to me that CEP couldn't work for guided weapons the way it did for unguided ballistic missiles. But I couldn't remember reading anything over the years about a different measurement of accuracy for precision munitions. So knowing that there isn't a different measure--but it works differently--actually makes me feel better.
UPDATE: Oh, and as long as we are talking acronyms, don't forget the problems with ROE (rules of engagement) that make even precision weapons less than effective. Do read it all, as the expression goes.
Smart bombs with dumb tactics is an expensive combination.