I have sent back so many Leupold's that I quit buying that brand. The most common issue I have found in all makes of scopes is the inability to hold a small group. Most start being able to shoot tight groups and then over time the groups get bigger and bigger with even the same lot of ammo. Change scope and the rifle goes back to shooting small groups. I have found now when I have shooting issues with a rifle I automatically change scopes first before anything else.
I have talked to several scope engineers from various companies and asked them why this happens. Still haven't got an answer.
Its fairly simple really, you have a whole bunch of mating parts that have to able to slide and turn and yet magically have zero clearance or backlash, which is something mostly impossible within the given space of a scope, and cost of manufacturing. Then some of these components will wear over time with use, good material selection, finishes, testing, etc will improve that, and it all costs $.
Same with all the machining, everything has tolerances, with the majority of scopes being mass produced very cheaply, there's no time to mix and match parts for best fit and so on. Scope #1000 could have been put together with a set of parts that were all ideal and makes a scope that will hold under 1/8, and #1001 had parts that(or even just 1 part) that pushed the tolerances and it'll be lucky to hold say 1/2moa, if it doesn't have other issues.
Now most people would never know their scope can only hold say 1/2moa on 99.9% of the rifles sold in this world, and they won't know they have a bad scope until the reticle falls out of it or similar major break down. But when you're pushing the limits in BR and F-class and some military use, its becoming a real thing and concern, along with much better testing on and off the gun. BR figured it out long ago I think, fixed/frozen scopes, minimum moving parts usually helps. Same with not making more adjustments than necessary while shooting.
As to vertical changes some mentioned, unless you're darn sure you're scope tracks perfect. If you're only off a click up or down and its holding, I now think its often best to aim off just a bit than to risk a jump. Put a click and it moves a minute... had it happen, seen it happen with others too. I think that gets worse near the ends of travel with some design, and again every brand has some differences in how they move the parts in there.